Add `select`, `get_character_rect` and `hit_test` to `RichTextLabel` #16029

Job	Run time
📊 Static checks / Code style, file formatting, and docs	33s
🐧 Linux / Editor w/ Mono (target=editor)	34m 1s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	36m 3s
🌐 Web / Template (target=template_release)	1m 28s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	31m 39s
🤖 Android / Template (target=template_release)	3m 38s
🍏 iOS / Template (target=template_release)	2m 57s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	9m 28s
🐧 Linux / Template w/ Mono (target=template_release)	1m 9s
🍎 macOS / Editor (target=editor, tests=yes)	41m 13s
🐧 Linux / Minimal template (target=template_release, everything disabled)	2m 0s
🍎 macOS / Template (target=template_release)	29m 0s
🏁 Windows / Editor (target=editor, tests=yes)	39m 57s
🏁 Windows / Template (target=template_release)	28m 29s
🪲 Godot CPP / Build and test Godot CPP	6m 42s
	4h 28m 17s

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