feat: Make file dropzone design tokens public and themeable

[avinashbot]

Description

Seems like the dropzone tokens inexplicably make up a notable chunk of the unthemeable tokens in our system. Check with design/design technologists and can't find a good reason not to expose these.

Closes #3744.

How has this been tested?

Updated snapshot tests.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.01%. Comparing base (9524306) to head (b361f81).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3753   +/-   ##
=======================================
  Coverage   97.01%   97.01%           
=======================================
  Files         825      825           
  Lines       23955    23963    +8     
  Branches     8420     8427    +7     
=======================================
+ Hits        23240    23248    +8     
  Misses        666      666           
  Partials       49       49           

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