docs: add status badges and an install table to the README

[fi3ework]

Summary

• Add a badge row under the title (CI, VS Marketplace version, Open VSX version, license), styled like the Rsbuild README (flat-square, 564341 / EDED91).
• Add an Install table linking the VS Code Marketplace and Open VSX listings, with per-registry version and installs/downloads badges.

Notes

• shields.io's visual-studio-marketplace/* badges are retired (they render as "retired badge"), so the two VS Marketplace badges use vsmarketplacebadges.dev, which supports the same style / label / labelColor / color parameters. Open VSX, CI and license badges stay on shields.io.

Test plan

• pnpm fmt:check
• pnpm lint
• Badges render on the PR's rendered README

[chatgpt-codex-connector]

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