Change default paths for storing the caches

[JDGrimes]

Currently they are wp-l10n-validator.cache and wp-l10n-validator-ignores.cache by default. I think this should be changed to .wp-l10n-validator-cache.json and .wp-l10n-validator-ignores-cache.json.

The benefit of making them .json is that it makes it more obvious what type of files they are (which could be useful for some dev tools). Prefixing with a . to make them hidden makes sense, as they really don't need to be cluttering up the root of a project.

We'd probably include back-compat for the current defaults.

I guess we could even combine these two files at some point, but that's a separate issue.