Add FINDFILTER⸺ variables

There are two possible approaches to how these variables could work :—

- By filtering _during_ the `find` (i·e using `-exec`), or
- By filtering _after_ the `find`.

The latter generally seems more efficient and the former doesn’t seem
to offer any benefits, so that is the approach taken here. Filters are
separated by semicolons, which are otherwise forbidden in file·names,
so it shouldn’t be a problem that the regular expressions won’t be able
to contain literal semicolons themselves.

The hope is that these variables provide a much cleaner solution to
`EXTRAFINDRULES` and `EXTRAFINDINCLUDERULES` hackery. Those are still
needed (for now) to manage two‐stage builds, but they’re cumbersome and
probably aren’t ideal in most cases.