feat(chips): Handle multi-select for filter chips

[bonniezhou]

Add multiple selection behavior for filter chips.

Follow-up PRs will address #2295 and #2296.

[codecov-io]

Codecov Report

Merging #2297 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2297      +/-   ##
==========================================
+ Coverage   99.04%   99.07%   +0.02%     
==========================================
  Files          99       99              
  Lines        3999     4004       +5     
  Branches      513      515       +2     
==========================================
+ Hits         3961     3967       +6     
+ Misses         38       37       -1

Impacted Files	Coverage Δ
packages/mdc-chips/chip-set/constants.js	100% <ø> (ø)	⬆️
packages/mdc-chips/chip-set/foundation.js	100% <100%> (+3.57%)	⬆️

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[kfranqueiro]

This LGTM, but...are filter chips supposed to be removable eventually? We might have to handle that carefully to make sure stale chips don't end up in the activeChips array...

[bonniezhou]

Yep, we also have to make sure that stale chips are removed from the chip-set's chips array.

@kfranqueiro I'm maybe rethinking the design of this... what do you think about adding a selected field to each chip and a selectedChoiceChip field to the chip-set, rather than having the chip-set hold a selectedChips array? This means that querying the selected chips would require looping through the entire chip-set's chips array, but the flip side is that it saves memory.
Though I'm not sure how much time/memory either option is really saving, so I think it comes down to what is best practice?

[kfranqueiro]

We discussed offline and decided to stick with the selectedChips array, figuring that the number of total chips is more likely to far outnumber the number of selected chips to the point where this may be ultimately more efficient. Moreover, having the separate array may be helpful for eventually providing information on selected chips in events.