CI: build Neva LSP binaries and package VSIX

[emil14]

Summary

• add GitHub Actions CI workflow to build all required neva-lsp binaries from nevalang/neva (darwin/linux/windows x amd64/arm64)
• upload each built binary as an artifact and then assemble them into bin/ for extension packaging
• build and package the extension (npm ci, npm run build, npx vsce package) and upload .vsix as CI artifact
• document CI behavior in CONTRIBUTING: no git submodule required for LSP

Why

This makes extension CI/CD self-contained: it can build the exact LSP binaries needed for release packaging without relying on manual local copying into bin/.

Notes

• workflow supports manual dispatch with optional neva_ref input to build against a specific nevalang/neva branch/tag/SHA
• no version bump or publishing changes included

Verification

• local: npm run build passes