types.optional (#114)

* inital commit

* dot

* Update docs/types-library-optional.md

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Co-authored-by: ariel <aweiss@hey.com>

docs/types-library-optional.md

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+# types.optional
+
+## Summary
+
+This RFC proposes adding a new method to the `types` library in type functions, that provides a shorthand for `types.unionof(type, types.singleton(nil))`.
+
+## Motivation
+
+Optional types occur very commonly, and it is annoying to have to always type `types.unionof(meow, types.singleton(nil))` to make a optional type, this leads to having the following appear in codebases that make use of optional types a lot in type functions:
+
+```luau
+type function optional(type)
+	return types.unionof(type, types.singleton(nil))
+end
+```
+
+## Design
+
+As a solution, a new method to the `types` library should be added called `optional`. This new method will act the same as `types.unionof(type, types.singleton(nil))`.