Warn on invalid @value definitions

[cmrn]

Just noticed a flaw with my previous PR #56 - RegExp.prototype.exec will return null if no match is found. Previously this wouldn't cause an error since the while statement would skip if matches was null. Now it will throw a TypeError: object null is not iterable when destructuring the result.

This PR prevents this error, and instead emits a postcss warning if the @value statement couldn't be parsed. This matches the behavior of postcss-modules-values as described in this test.

[princed]

Very thorough approach, thank you!
Out as 4.2.2