chore(plugin-pnp): add enum NodeLinker

[trivikr]

What's the problem this PR addresses?

Fixes: #5847
Previous attempt: #5070

How did you fix it?

Add enum NodeLinker which can used wherever nodeLinker configuration is used.

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.

[merceyz]

Similarly to my objection in #5070 (review), I don't like that the PnP linker has to "know about" the others and vice versa.

I'd be fine with just updating the description but in the long run I think this would be better solved by something along the lines of:

1. a @yarnpkg/plugin-node plugin that only registers the nodeLinker configuration with a description
2. the linkers register the nodeLinker configuration only providing the valid value(s) for itself, a description for said value(s), and if any of them should be the default.
3. the core merges them and provides validation solving [Enhancement] Throw on invalid nodeLinker value #2656.

[trivikr]

I posted a fix to mention pnpm in nodeLinker description as of now #5850

The suggestion by @merceyz for creating @yarnpkg/plugin-node can be taken up separately.