Add `extension.vsixmanifest` file for executable VSIX bundles

[ProCode-Software]

Currently, Klar's VSCode extension has to be packed by vsce because vsxtools doesn't create an extension.vsixmanifest in the .vsix bundle that specifies that the extension has executable code. If packed by vsxtools, the commands and LSP won't run.

Excerpt:

<?xml version="1.0" encoding="utf-8"?>
	<PackageManifest Version="2.0.0" xmlns="http://schemas.microsoft.com/developer/vsx-schema/2011" xmlns:d="http://schemas.microsoft.com/developer/vsx-schema-design/2011">
		<Metadata>
			<Properties>
				<Property Id="Microsoft.VisualStudio.Code.ExecutesCode" Value="true" />

For extensions that contain only TextMate grammars, I haven't seen any issues, but I'll have to do further testing.