This adds support for formatters installed locally in project
directories via yarn 2's "zero install" [pnp
mode](https://yarnpkg.com/features/pnp).
It's quite similar to the support for formatters installed locally in a
project's `node_modules` via npm, and leverages the `npx` symbol, so
existing formatter definitions should work without modification.
This checks for a `.pnp.cjs` file (expected in the project root for yarn
pnp projects), then looks for a yarn executable, and checks the version
of yarn to make sure it supports pnp. If that works, we just push
`"yarn"` onto the front of `command`.
I've only tested this with a locally installed `prettier.js`. It's very
much a works-for-me draft, I'm putting in a PR to make sure this is a
workable approach before going any further with it.
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Co-authored-by: Radon Rosborough <radon@intuitiveexplanations.com>