Add VS Code extension for HyperDoc language support

[ikskuh]

Summary

• add a new vscode-ext workspace with a HyperDoc extension manifest and grammar registration
• implement syntax highlighting, language configuration, and context-aware completions for elements vs attributes
• scaffold a configurable wasm language server stub with a command to prepare the module without enabling it by default
• add a justfile to simplify setup, build, test, and packaging workflows for the extension
• refactor shared logic into testable utilities and add unit tests for completions, attribute detection, and wasm path resolution
• add a wasm language server stub build target and CI workflow that builds, tests, packages, and stages the VS Code extension with wasm artifacts, plus a protected publish job for the hdoc-2.0 branch

Testing

• zig build install
• zig build test
• npm run compile
• npm test

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Codex Task