RFC: Named Function Type Arguments (#36)

Introduces syntax for optional names of function type arguments.

rfcs/syntax-named-function-type-args.md

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+# Named function type arguments
+
+## Summary
+
+Introduce syntax for optional names of function type arguments.
+
+## Motivation
+
+This feature will be useful to improve code documentation and provide additional information to LSP clients.
+
+## Design
+
+This proposal uses the same syntax that functions use to name the arguments: `(a: number, b: string) -> string`
+
+Names can be provided in any place where function type is used, for example:
+
+* in type aliases:
+```
+type MyFunc = (cost: number, name: string) -> string
+```
+
+* in definition files for table types:
+```
+declare string: {
+    rep: (pattern: string, repeats: number) -> string,
+    sub: (string, start: number, end: number?) -> string -- names are optional, here the first argument doesn't use a name
+}
+```
+
+* for variables:
+```
+local cb: (amount: number) -> number
+local function foo(cb: (name: string) -> ())
+```
+
+Variadic arguments cannot have a name, they are already written as ...: number.
+
+This feature can be found in other languages:
+
+* TypeScript (names are required): `let func: (p: type) => any`
+* C++: `void (*f)(int cost, std::string name) = nullptr;`
+
+Implementation will store the names inside the function type description.
+
+Parsing the argument list will require a single-token lookahead that we already support.
+Argument list parser will check if current token is an identifier and if the lookahead token is a colon, in which case it will consume both tokens.
+
+Function type comparisons will ignore the argument names, this proposal doesn't change the semantics of the language and how typechecking is performed.
+
+## Drawbacks
+
+Argument names require that we create unique function types even when these types are 'identical', so we can't compare types using pointer identity.
+
+This is already the case in current Luau implementation, but it might reduce the optimization opportunities in the future.
+
+There might also be cases of pointer identity checks that are currently hidden and named arguments might expose places where correct unification is required in the type checker.