feat(text-field): Expand the helper text foundation

[bwobrien]

Adds the ability to dynamically set whether the helper text is persistent or whether it acts as a validation message.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@2c92827). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1955   +/-   ##
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  Coverage          ?   99.43%           
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  Files             ?       84           
  Lines             ?     3718           
  Branches          ?      486           
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  Hits              ?     3697           
  Misses            ?       21           
  Partials          ?        0

Impacted Files	Coverage Δ
packages/mdc-textfield/helper-text/foundation.js	94.59% <100%> (ø)

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[lynnmercier]

LGTM.

Does this relate to #1590? Maybe you can update that issue with more information from design?

[bwobrien]

No, this does not change behavior as described in issue #1590. I will ping design about that one.