avm2: Implement TextField.selectedText #22730

Job	Run time
Paths filter	9s
Check dependencies	1m 30s
Lints with Rust stable	4m 43s
Lints with Rust beta	4m 50s
Test Rust stable / ubuntu-22.04	4m 34s
Test Rust ${{ matrix.rust_version }} / ${{ matrix.os }}	0s
Lints with Rust nightly	4m 50s
Test Rust stable / windows-latest	11m 15s
Test Rust stable / macos-14	3m 53s
Test Rust nightly / ubuntu-22.04	4m 39s
Test Rust beta / ubuntu-22.04	4m 26s
	44m 49s

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