fix(list): Update clickable elements selector

[williamernest]

Updates the enabled items selector to search for descendant elements of .mdc-list-item instead of all a and button elements.

Updates Readme demo to show tabindex=0 for the first element that should be in tab order for single selection lists.

Updates the list component to default to vertical list if aria-orientation is not present or is misspelled.

Adds unit test for instantiating the component with various aria-orientation values.

[mdc-web-bot]

All 188 screenshot tests passed for commit 93c8546 vs. master! 💯🎉

[codecov-io]

Codecov Report

Merging #3312 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3312   +/-   ##
=======================================
  Coverage   98.03%   98.03%           
=======================================
  Files         120      120           
  Lines        5184     5184           
  Branches      649      649           
=======================================
  Hits         5082     5082           
  Misses        102      102

Impacted Files	Coverage Δ
packages/mdc-list/constants.js	100% <ø> (ø)	⬆️
packages/mdc-list/index.js	98.78% <100%> (ø)	⬆️

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[abhiomkar]

LGTM.

[mdc-web-bot]

All 188 screenshot tests passed for commit 6c31158 vs. master! 💯🎉