refactor(utils): update slotMutationController to allow multiple slot names

[averyjohnston]

Issue number: Internal

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What is the current behavior?

The internal slotMutationController utility can only watch one slot name at a time.

What is the new behavior?

Added the option to specify multiple slot names, any of which will trigger the callback. This change is made in preparation for upcoming start and end slots for several form control components; it boosts performance by preventing us from needing to add multiple createSlotMutationController calls to each component.

Does this introduce a breaking change?

• Yes
• No

Other information

[thetaPC]

LGTM

[liamdebeasi]

Code looks good, but this should be a refactor(util) commit since it's an internal change.

[averyjohnston]

Ah, right, perf will show up in the changelog, huh 🤔 I figured refactor didn't fit since this is a behavior change (if an internal one) but it makes sense if the concern is more public API changes.