fix: Fix forwardRef warning in split panel

[just-boris]

Description

Replace forwardRef with a plain component to fix this React warning:

Function components cannot be given refs. Attempts to access this ref will fail. Did you mean to use React.forwardRef()

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

How has this been tested?

Locally, because it is only a warning

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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[just-boris]

That was the last use of this function. Remove it, because we do not support forward-ref-as-widgets

[codecov]

Codecov Report

Attention: Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.20%. Comparing base (ce7dff3) to head (53cf6c5).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
src/split-panel/implementation.tsx	98.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2884      +/-   ##
==========================================
- Coverage   96.20%   96.20%   -0.01%     
==========================================
  Files         761      761              
  Lines       21445    21434      -11     
  Branches     7340     7326      -14     
==========================================
- Hits        20631    20620      -11     
  Misses        761      761              
  Partials       53       53              

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