Implements the compat plugin

[arcanis]

What's the problem this PR addresses?

In order to improve the compatibility with third-party tools, we want to provide builtin patches to some high-values projects (the only one at this time being TypeScript). Those patches should be kept within their own plugins so that they don't leak within the Yarn codebase itself, and should stay minimal.

Ref #589

How did you fix it?

• The patch plugin now supports a new hook called getBuiltinPatch. This hook will get triggered when the patch specifier is of the form builtin<something>, with something being passed in parameter. The hook will then be able to return the adequate patch content.

• The core now supports a new hook called reduceDescriptorAlias. This hook will get triggered once for each unique dependency in the project, and is expected to return a new descriptor (either the original one, or a wrapper around the original one).

• A new plugin called compat implements both of these hooks: the typescript dependencies are converted into patch:typescript@...#builtin<compat/typescript>, which then applies the stock transform on the original TypeScript package.

• The script used to generate the typescript patchfile is currently stored within the compat package. It just requires to clone my TypeScript fork so that mael/pnp is available.

[orta]

I like it, it's a miniature copy of patch-package in Yarn :D