Add a check to ensure that all filters have the executable bit set

[pmolodo]

Pandoc has a requirement that it's filters be executable. When creating them, though, it is easy to forget to set this bit.

Add a github action that confirms this is set, and/or a pre-commit hook.

Note: if adding a pre-commit hook, recommend installation via prek, to unify handling with aousd/core-spec-supplemental:

• https://github.com/aousd/core-spec-supplemental/pull/170/changes
• https://github.com/aousd/core-spec-supplemental/pull/190/changes