Add RichTextLabel `is_finished()`, deprecate `is_ready()` #652

Job	Run time
📊 Static checks / Code style, file formatting, and docs	1m 7s
🍏 iOS / Template (target=template_release)	10m 22s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	24m 53s
🤖 Android / Editor (target=editor)	25m 19s
🌐 Web / Template w/o threads (target=template_release, threads=no)	25m 8s
🐧 Linux / Editor w/ Mono (target=editor)	40m 9s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	16m 53s
🐧 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)	1h 17m 42s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	17m 24s
🏁 Windows / Editor (target=editor, tests=yes)	48m 24s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	37m 19s
🍎 macOS / Editor (target=editor, tests=yes)	30m 49s
🏁 Windows / Template (target=template_release)	35m 35s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	32m 33s
🍎 macOS / Template (target=template_release)	23m 3s
🐧 Linux / Template w/ Mono (target=template_release)	26m 24s
🐧 Linux / Minimal template (target=template_release, everything disabled)	14m 17s
🪲 Godot CPP / Build and test Godot CPP	8m 12s
	8h 15m 33s

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