feat(Input): add placeholderColor variable for Teams theme

[layershifter]

Fixes #1036.

<Input placeholder="Search..." variables={{ placeholderColor: 'red' }} />

[codecov]

Codecov Report

Merging #1092 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1092   +/-   ##
=======================================
  Coverage   82.17%   82.17%           
=======================================
  Files         701      701           
  Lines        8596     8596           
  Branches     1251     1251           
=======================================
  Hits         7064     7064           
  Misses       1516     1516           
  Partials       16       16

Impacted Files	Coverage Δ
...rc/themes/teams/components/Input/inputVariables.ts	100% <ø> (ø)	⬆️
...t/src/themes/teams/components/Input/inputStyles.ts	8.33% <ø> (ø)	⬆️

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