feat: FileInput component (internal)

[katiegeorge]

Description

This PR is part one of multiple for file upload decomposition, to make the pull requests easier to review.

Full plan:

• File input (internal) - this PR
• File token group (internal)
• File dropzone (internal)
• PR to move all 3 out of /internal to make public
• Adding file-upload variant to button group

Related links, issue #, if available: n/a

How has this been tested?

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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[katiegeorge]

For reviewer: is this how we typically enforce a property that is only required in certain configurations? Should we throw an error instead?

[codecov]

Codecov Report

Attention: Patch coverage is 92.59259% with 4 lines in your changes missing coverage. Please review.

Project coverage is 96.23%. Comparing base (6ac2909) to head (048b8df).

Files with missing lines	Patch %	Lines
src/internal/components/file-input/index.tsx	90.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #2912    +/-   ##
========================================
  Coverage   96.23%   96.23%            
========================================
  Files         767      768     +1     
  Lines       21699    21720    +21     
  Branches     7363     7028   -335     
========================================
+ Hits        20882    20903    +21     
- Misses        764      809    +45     
+ Partials       53        8    -45     

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