Make `Filter`s public

[nicholasspencer]

Is there a particular reason the filters are scoped internal? It would be nice if they were public to use the filters directly in other customs filters.

[nicholasspencer]

I have a fork that scopes the filters publicly but i'm wondering if it would be useful to move the filters into their own target which could be used by StencilSwiftKit and other gen tools.

I know that I should be using macros for most of this stuff but there are certain situations where using both macros and custom filters would be nice.

[djbe]

No particular reason TBH, beyond the fact we didn't think about it 😆
This is a perfect starter task, if you want to make a PR.

Note: you can get started on it now if you really want, but might be best to wait until #127 is merged (which modernises lots of things).