Add sigmoid(), sigmoid_approx(), sigmoid_affine() and sigmoid_affine_approx() functions #101

Job	Run time
📊 Static checks / Code style, file formatting, and docs	23s
🍏 iOS / Template (target=template_release)	1m 55s
🤖 Android / Editor (target=editor)	4m 49s
🐧 Linux / Editor w/ Mono (target=editor)	3m 20s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	3m 21s
🐧 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)	12m 28s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	11m 38s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 22s
🍎 macOS / Editor (target=editor, tests=yes)	3m 51s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	8m 42s
🍎 macOS / Template (target=template_release, tests=yes)	3m 2s
🐧 Linux / Template w/ Mono (target=template_release, tests=yes)	2m 58s
🐧 Linux / Minimal template (target=template_release, tests=yes, everything disabled)	2m 30s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 13s
🌐 Web / Template w/o threads (target=template_release, threads=no)	7m 13s
🏁 Windows / Editor (target=editor, tests=yes)	8m 38s
🏁 Windows / Editor w/ clang-cl (target=editor, tests=yes, use_llvm=yes)	3m 57s
🏁 Windows / Template (target=template_release, tests=yes)	7m 19s
🪲 Godot CPP / Build and test Godot CPP	8m 35s
	1h 44m 14s

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