feat: Add @sentry/bundler-plugins core package

[timfish]

Consolidates all bundler plugin code into a single new @sentry/bundler-plugins package with subpath exports (/rollup, /webpack, /vite, etc.). Code was moved using git mv to preserve history.

The existing bundler-specific packages (@sentry/rollup-plugin, etc.) become thin stubs that re-export from @sentry/bundler-plugins.

This makes it straightforward to:

• Add new bundler support (e.g. rolldown) without publishing a new package
• Move all plugin code in the JS monorepo as a single dependency rather than 6
• Keep bundler-specific packages at v5, avoiding unnecessary major version bumps for users

Individual packages can eventually be deprecated but there's no urgency — integration tests still target them and they remain fully functional.

[cursor]

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[isaacs]

👍