feat: editor plugin Phase 1: VSCode extension foundation (grammar + bundled tsserver plugin + packaging)

[vivek7405]

Problem

Phase 1 of #381. webjs has no first-class editor extension: highlighting inside html``/css`` templates needs the third-party lit-html VSCode extension, and the tsserver features live in @webjsdev/ts-plugin which the user must wire into tsconfig. Ship a single webjs VSCode extension (also installable in Cursor / Antigravity / Windsurf via Open VSX) that gives the complete experience out of the box.

Design / approach

New monorepo package packages/vscode, extension name webjs (marketplace id webjsdev.webjs):

• Own TextMate injection grammar so html``/css (and `svg) tagged-template contents are highlighted as HTML/CSS WITHOUT the lit-html extension. Adapt the injection grammars from the local lit/lit clone (packages/labs/vscode-extension/syntaxes/), which target the same html/css tag names webjs uses; ship our own copy so there is no Lit-extension dependency.
• Auto-register the tsserver plugin via contributes.typescriptServerPlugins pointing at the bundled @webjsdev/ts-plugin, with enableForWorkspaceTypeScriptVersions so it loads with no tsconfig edit. (Still bundles ts-lit-plugin under the hood at this phase; the parity rebuild + Lit-dep removal are epic: all-in-one webjs editor plugin (VSCode + Neovim), retire ts-plugin + Lit dep #381 phases 2-3.)
• Snippets for the common recipes (page, dynamic route, server action, component, module), mirroring the MCP prompts.
• Commands: run webjs check, scaffold (webjs create), open docs.
• Packaging: vsce package (VS Marketplace) + ovsx publish (Open VSX, which Cursor / Antigravity / Windsurf / VSCodium pull from). Icon, README, LICENSE, engines.vscode, categories/keywords for discoverability.

Acceptance criteria

• Installing the extension highlights html``/css/`svg template contents (HTML/CSS tokens) with NO Lit extension present.
• The webjs tsserver features (go-to-def, attribute autocomplete, register('tag') diagnostic suppression) work without the user editing tsconfig (auto-registered by the extension).
• Snippets + commands are available; the extension activates on .ts/.js webjs files.
• vsce package produces a valid .vsix; an ovsx-compatible package is producible; manifest declares publisher/categories/keywords/icon for marketplace discoverability.
• Tests: grammar tokenization test (an `html``` block tokenizes as HTML), and a manifest/contributions validation test.
• Docs: packages/vscode/README.md (marketplace-facing) + a note in the root AGENTS.md / cli AGENTS.md pointing at the extension; the webjs mcp and ts-plugin docs cross-reference it.