feat(menu): Support default focus state

[trimox]

• Adds defaultFocusState: 'none' | 'list' | 'firstItem' | 'lastItem' for MdcMenu
• Minor code refactoring of MdcMenuSurfaceBase
• Update documentation
• Include unit test

closes #1998

[codecov]

Codecov Report

Merging #2004 into master will increase coverage by 0.31%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #2004      +/-   ##
==========================================
+ Coverage   90.33%   90.64%   +0.31%     
==========================================
  Files          97       97              
  Lines        4302     4307       +5     
  Branches      578      579       +1     
==========================================
+ Hits         3886     3904      +18     
+ Misses        219      206      -13     
  Partials      197      197

Impacted Files	Coverage Δ
packages/menu/menu.ts	87.95% <88.88%> (+16.15%)	⬆️
packages/list/list.ts	84.86% <0%> (+0.45%)	⬆️

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