feat(slugs): allow filters in template strings

[stormwarning]

Process template filters first, then strip ' and . characters from slug strings.

Fixes #4783

Summary

Provides greater control over field values used in entry slug strings.

Test plan

Added a new formatters test which failed under the existing setup. Updated compileStringTemplate() function such that new test and all existing tests pass.

Checklist

• [x] I have read the contribution guidelines.
• [x] Code is formatted via running yarn format.
• [x] Tests are passing via running yarn test.
• [ ] The status checks are successful (continuous integration). Those can be seen below.

[stormwarning]

Also happy to update the docs if the rest of the code looks good 👍

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[jenniferbhampton]

Could really use this feature, hopefully it gets approved and merged!

[nhoizey]

If this allows use setting for example our own slug filter, it would be awesome.

For example:

slug: {{title | my_own_slugify}}