feat: allow multiple filter values, fixes #7328

[paulroub]

Summary

Allows a collection filter to match one of a number of values; still allows the existing single-value syntax, as well as an array of values.

Need to manage a collection that's a combination of existing categories (rather than re-categorize all those posts).

Test plan

• Added a unit test for the array variation, existing (non-array) tests still path.
• Tested against my collections, with single-value, single-element-array and multiple-element-array variations all working as expected.

Checklist

Please add a x inside each checkbox:

• [X] I have read the contribution guidelines.
• [X] I have added a documentation update PR at https://github.com/decaporg/decap-website/pull/92

A picture of a cute animal (not mandatory but encouraged)

[martinjagodic]

@paulroub, thanks for your contribution. Before we review, can you please ensure the tests are passing?

[paulroub]

@martinjagodic Fixed the linting issues, along the way had to tell stylelint to ignore backend.ts as it was trying to parse a cast as CSS and was not happy with the result. No actual styles in this file, of course, so that should be harmless.