Adds basic support for .yamlrc.yml (yarnPath only, v2)

[arcanis]

Summary

Ref: yarnpkg/berry#239

I'm planning to rename .yarnrc files into .yarnrc.yml in order to avoid problems where configuration for two different syntaxes / documentations would have to be used together. My hope is that it'll help the migration plan become smoother and less unpredictable.

This proposal has one drawback: the yarn-path settings. The v1 (which I expect to stay the version in the global path for some time) doesn't know how to read the .yarnrc.yml file, and to look for the yarn-path settings there. We could make the v2 write the yarn-path settings into the old .yarnrc file (rather than .yarnrc.yml), but that would require to have both .yarnrc and .yarnrc.yml at the same time when using the v2, which I'd prefer to avoid.

To solve this, my plan is to:

1. continue to make the v1 reading the .yarnrc files
2. update the v1 to also look for yarn-path in the .yarnrc.yml file
3. make the v2 exclusively read/write from the .yarnrc.yml file
4. update the v1 to make yarn policies set-version write a .yarnrc.yml when installing the v2

This diff solves 1, 2, and 3. The fourth point is optional and will be done at a later point if at all.

Test plan

Ran Yarn with a local .yarnrc.yml, worked successfully. Tests will validate that the old workflow also work.