Add official VS Code language extension

[konojunya]

Summary

• register .stack files with the canonical grammar and language configuration from @stack-sh/language
• launch stack lsp only after a shell-free CLI version probe, with trusted-workspace and machine-scoped configuration boundaries
• provide diagnostics, completion, hover, symbols, formatting, lifecycle status, recovery guidance, and a Playground command
• add allowlisted VSIX packaging, third-party notices, SHA-pinned CI, package attestation, and OIDC Marketplace release automation

Compatibility

• Visual Studio Code >=1.91.0
• Stack CLI >=0.4.0 <0.5.0
• @stack-sh/language 0.1.0

Verification

• npm run check
• npm run audit (0 vulnerabilities)
• actionlint .github/workflows/*.yaml
• npm run test:integration (5 tests on VS Code 1.91.0 and current stable, using the inspected VSIX and Stack CLI 0.4.0)
• VSIX allowlist, canonical asset byte comparison, and SHA-256 inventory