Store the LESS cache in options, rather than a file

This moves the LESS cache from the filesystem to options.

When storing LESS externally (such as on S3), the filesystem is tens of orders of magnitude slower than the database. In any case, storing the cache on the filesystem is typically a bad idea, as it means extra filesystem reads when they're not necessary.

With the LESS cache in options, this benefits from things like WP's object cache (much faster), and option autoloading, which makes the cache check basically free.

Props to @joehoyle for fixing this on the Human Made fork; I simply rebased the changes onto the newer WP-LESS versions.