feat(core): publish the livekit-client/core entry
`src/exports.ts` is the export surface both entries share. The main entry
adds `Room` and the type augmentations; the core entry adds `CoreRoom` and
the extension objects. The core entry is built per module with dependencies
left external (dist/core/), so a consumer's bundler drops unused modules
whole; a single-file core bundle was not tree-shakeable by esbuild. The
connection checker builds its own room from CoreRoom so that the full Room
never reaches the core graph.
Also adds examples/core, a README section, and size-limit entries for the
new entry.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
