GDScript: Allow utility functions to be used as `Callable` #18964

Job	Run time
📊 Static checks / Code style, file formatting, and docs	33s
🍏 iOS / Template (target=template_release)	11m 51s
🐧 Linux / Editor w/ Mono (target=editor)	26m 55s
🐧 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)	28m 45s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	26m 14s
🍎 macOS / Editor (target=editor, tests=yes)	24m 25s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	26m 26s
🐧 Linux / Template w/ Mono (target=template_release)	17m 1s
🍎 macOS / Template (target=template_release)	21m 35s
🤖 Android / Template (target=template_release)	21m 0s
🐧 Linux / Minimal template (target=template_release, everything disabled)	11m 38s
🌐 Web / Template (target=template_release)	13m 17s
🏁 Windows / Editor (target=editor, tests=yes)	32m 19s
🏁 Windows / Template (target=template_release)	21m 13s
🪲 Godot CPP / Build and test Godot CPP	6m 54s
	4h 50m 6s

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