Add match: MuteWordMatch to muted-word mod decision cause

[estrattonbailey]

This PR adds a match: MuteWordMatch prop to the muted-word cause type. Previous logic was checking for truthy/falsy values, so the migration was pretty straightforward. This requires no change to the matching logic itself.

Matching logic now:

• returns MuteWordMatch | null instead of a boolean
• migrates some tests to toBeTruthy and toBeNull
• adds the new prop to muted-word cause
• updates a few tests to check for this new prop

[estrattonbailey]

Leaving this open ended in case we wanted to add additional props to the match in the future.