Add autocompletion for `OS.has_feature()` #181

Job	Run time
📊 Static checks / Code style, file formatting, and docs	2m 44s
🤖 Android / Template (target=template_release)	27m 2s
🍏 iOS / Template (target=template_release)	2m 28s
🍎 macOS / Editor (target=editor, tests=yes)	5m 56s
🐧 Linux / Editor w/ Mono (target=editor)	4m 19s
🍎 macOS / Template (target=template_release)	15m 26s
🌐 Web / Template (target=template_release)	18m 40s
🐧 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)	17m 35s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	7m 7s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	6m 44s
🏁 Windows / Editor (target=editor, tests=yes)	8m 59s
🐧 Linux / Template w/ Mono (target=template_release)	25m 58s
🏁 Windows / Template (target=template_release)	29m 43s
🐧 Linux / Minimal template (target=template_release, everything disabled)	13m 36s
🪲 Godot CPP / Build and test Godot CPP	6m 49s
	3h 13m 6s

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