editor: Add classic text search functionality to Output panel

Implements classic search in the Output (EditorLog) panel, including:
- Search box with toggles for "Match Case" and "Whole Words"
- Navigation buttons for next/previous match
- Highlights matches, with distinction for the currently selected
- Scroll to active match when navigating

This improves usability for developers debugging or scanning log
output by allowing quick and flexible keyword searching.
This looks to implement what was referenced in godot-proposals
features  #12074 and #10249.
Co-authored-by: Sun Chenwei <sunchenwei@tecnico.ulisboa.pt>

Author: yangyey