This PR removes the manual vendoring process. Third party code can now
be updated using the typical NPM pipeline with types/code bundling done
through Rollup.
* chore: vendor Mitt into src/common/third-party
As discussed in #6203 we need to vendor our common dependencies in so
that when we ship an ESM build all imports point to file paths and do
not rely on Node resolution (e.g. a browser does not understand `import
mitt from 'mitt'`).
