Add improvements to text editor

[ahuang11]

Description

Adds a few params, namely:

value_input to differentiate from value

selection to capture what the user is selecting (and potentially have the LLM rewrite that portion)

on_keyup to prevent value from being value_input and to prevent breaking changes; mirrors CodeEditor

How Has This Been Tested?

abcdefgg.mp4

AI Disclosure

• This PR contains AI-generated content.
  • I have tested all AI-generated content in my PR.
  • I take responsibility for all AI-generated content in my PR.

Tools and Models: Opus 4.7 for planning and implementation

Checklist

• Tests added and are passing
• Added documentation

[codecov]

Codecov Report

❌ Patch coverage is 17.39130% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.08%. Comparing base (d7f8e7a) to head (ff2108f).

Files with missing lines	Patch %	Lines
panel/tests/ui/widgets/test_texteditor.py	0.00%	38 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (d7f8e7a) and HEAD (ff2108f). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (d7f8e7a)	HEAD (ff2108f)
	7	6

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #8560       +/-   ##
===========================================
- Coverage   86.25%   70.08%   -16.18%     
===========================================
  Files         350      349        -1     
  Lines       55523    55566       +43     
===========================================
- Hits        47891    38941     -8950     
- Misses       7632    16625     +8993     

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