In `src/common` we now use `fs.promises.X` which we can dynamically
`import`. In a browser environment this code will never run because it's
gated on `isNode` (in a future PR we will add tree-shaking to the bundle
step such that this code is eliminated). By using `import`, we ensure
TypeScript still can track types and give good type information.
In `src/node` we continue to use `util.promisify` but that's not a
concern as that code explicitly is never run in the browser.
* 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'`).
