chore(vscode): use `unjs/unbuild` for bundling

[kermanx]

Use https://github.com/unjs/unbuild to bundle the VSCode extension, also as a preparation for #4994.

Notes:

• To dev the extension: run pnpm dev:vue, the command will exit after the stub files generated (no need to watch file changes), then F5. (Should the tasks defined in .vscode/tasks.json and .vscode/launch.json be restored?)
• To build the extension: like before.
• Other packages except the VSCode extension remain unchanged.

[pkg-pr-new[bot]]

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commit: 8eaeb65

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