feat: add folder upload support to FileInput and FileDropzone

[kevingomes17]

Description

This PR adds folder upload support to the FileInput and FileDropzone components, enabling users to select and upload entire folders of files.

Key changes:

FileInput Component:

• Added new mode prop ('file' | 'folder') to enable folder selection
• When mode='folder', the component renders with the webkitdirectory attribute
• Folder mode automatically enables multiple file selection regardless of the multiple prop
• Files are filtered by accept criteria after selection (since native accept filtering doesn't work with webkitdirectory)

FileDropzone Component:

• Added new accept prop to filter dropped files by type
• Enhanced onDrop handler to recursively traverse dropped folders using DataTransferItem.webkitGetAsEntry() API
• Supports mixed drops of files and folders, combining all files into a single array

Shared Utilities:

• Created accept-filter.ts with file type filtering functions supporting extensions (.jpg, .png) and MIME types (image/*)
• Created folder-traversal.ts with async folder traversal utilities

Type Declarations:

• Added TypeScript declarations for webkitdirectory attribute and FileSystem API types in
  global.d.ts
• Updated component definitions for documentation

Related links, issue #, if available: https://quip-amazon.com/0y0pAIzFG0mD/Folder-support-for-file-input-Tech-design

How has this been tested?

• Unit tests pass for all modified components
• Snapshot tests updated for component definitions
• Manual testing of folder selection in FileInput with mode='folder'
• Manual testing of folder drag-and-drop in FileDropzone

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

❌ Patch coverage is 30.30303% with 92 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.81%. Comparing base (e7feb35) to head (c1ef84d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/internal/utils/accept-filter.ts	6.12%	41 Missing and 5 partials ⚠️
src/internal/utils/folder-traversal.ts	35.71%	44 Missing and 1 partial ⚠️
src/file-input/internal.tsx	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4207      +/-   ##
==========================================
- Coverage   97.16%   96.81%   -0.35%     
==========================================
  Files         879      881       +2     
  Lines       25749    25877     +128     
  Branches     9303     9338      +35     
==========================================
+ Hits        25018    25054      +36     
- Misses        725      770      +45     
- Partials        6       53      +47     

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