Also find [`pnpm`] package.yaml/json5 properties

[danielbayley]

pnpm supports both package.yaml, and package.json5 (see https://github.com/pnpm/pnpm/pull/1799). It would be a helpful nudge for the ecosystem if cosmiconfig could also find properties in these files, as it currently does package.json. It already supports reading from YAML, so I imagine this to be fairly low hanging fruit…

[d-fischer]

package.yaml was added for the new alpha (publishing some time today), JSON5 is not currently supported out of the box. You may add package.json5 to your searchPlaces and a JSON5 loader to your loaders to support this for now, as the code now checks for a package file with any extension.

Before adding more loaders to ship out of the box, I want to implement #333 first.