ci: measure test coverage and publish it to Coveralls

[nigrosimone]

Only pg-connection-string measures its test coverage today. For the other packages there is no way to see which code the tests do not touch.

This adds a root yarn coverage script that runs the whole test suite under nyc and prints one report for all packages.

---------------------------|---------|----------|---------|---------|-------------------------------
File                       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s             
---------------------------|---------|----------|---------|---------|-------------------------------
All files                  |   90.37 |    84.33 |   89.27 |   90.88 |                               
 pg-cloudflare/src         |   38.88 |    41.02 |   37.03 |      39 |                               
  empty.ts                 |       0 |        0 |       0 |       0 |                               
  index.ts                 |   38.88 |    41.02 |   37.03 |      39 | ...23,130-148,162-166,173-181 
 pg-connection-string      |     100 |      100 |     100 |     100 |                               
  index.js                 |     100 |      100 |     100 |     100 |                               
 pg-cursor                 |    95.9 |    88.88 |   96.15 |   95.65 |                               
  index.js                 |    95.9 |    88.88 |   96.15 |   95.65 | 204-208,252                   
 pg-native                 |   88.44 |    68.34 |   93.02 |   90.87 |                               
  index.js                 |   88.44 |    68.34 |   93.02 |   90.87 | ...61,370-371,405,418,449-452 
 pg-native/lib             |   84.49 |    59.52 |   86.95 |   85.24 |                               
  build-result.js          |     100 |      100 |     100 |     100 |                               
  copy-stream.js           |    75.6 |    52.77 |      80 |   77.21 | ...07,120-121,128,134,143,147 
 pg-pool                   |   96.72 |     93.5 |   98.33 |   96.59 |                               
  index.js                 |   96.72 |     93.5 |   98.33 |   96.59 | ...14,169,257-260,354,361,457 
 pg-protocol/src           |   99.43 |    91.45 |     100 |   99.41 |                               
  buffer-reader.ts         |     100 |      100 |     100 |     100 |                               
  buffer-writer.ts         |     100 |       90 |     100 |     100 | 53                            
  index.ts                 |     100 |      100 |     100 |     100 |                               
  messages.ts              |     100 |      100 |     100 |     100 |                               
  parser.ts                |   98.63 |     90.9 |     100 |   98.59 | 89,354,377                    
  serializer.ts            |     100 |     92.5 |     100 |     100 | 193,239-240                   
 pg-query-stream/src       |     100 |      100 |     100 |     100 |                               
  index.ts                 |     100 |      100 |     100 |     100 |                               
 pg/lib                    |   92.34 |    89.08 |   87.42 |   92.44 |                               
  client.js                |   89.09 |    86.41 |   84.33 |   89.04 | ...02,652-655,722,740,838-839 
  connection-parameters.js |   97.93 |    93.33 |     100 |   97.91 | 133,165                       
  connection.js            |   94.28 |    93.75 |   87.87 |   94.28 | 35,141,156,238-246            
  defaults.js              |     100 |    83.33 |     100 |     100 | 5                             
  index.js                 |   92.68 |       25 |     100 |   92.68 | 47,61-62                      
  query.js                 |   99.01 |      100 |   88.23 |   99.01 | 251                           
  result.js                |     100 |    91.66 |     100 |     100 | 37,101                        
  stream.js                |      50 |    35.71 |    62.5 |      50 | 41-49,69-75,80                
  type-overrides.js        |   94.44 |     90.9 |     100 |   94.44 | 18                            
  utils.js                 |   95.28 |    93.75 |     100 |      97 | 118-119,173                   
 pg/lib/crypto             |   71.42 |    59.83 |   92.85 |   72.72 |                               
  cert-signatures.js       |   42.85 |    18.91 |      80 |   43.54 | 2,9-17,61-63,67-119           
  sasl.js                  |   86.44 |    80.24 |     100 |   86.84 | ...05,211,232,243,246,249,252 
  utils.js                 |   81.81 |       25 |    87.5 |   81.81 | 42-45                         
 pg/lib/native             |   91.71 |    90.66 |   85.24 |   92.03 |                               
  client.js                |   91.94 |    88.88 |   89.36 |   92.07 | 9,168,211,359-369,404-407,419 
  index.js                 |     100 |      100 |     100 |     100 |                               
  query.js                 |   91.01 |    95.23 |   71.42 |   91.86 | 67,71,75-82                   
---------------------------|---------|----------|---------|---------|-------------------------------
Done in 172.91s.