RFC: Support Variadic User-Defined Type Functions

docs/variadic-user-defined-type-functions.md

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+# Variadic User-Defined Type Functions
+
+## Summary
+
+Add support for variadic arguments for user-defined type functions.
+
+## Motivation
+
+Today, user-defined type functions can only take a pre-defined set of parameters. This limits the usage in a way that makes it not possible to make the function variadic, where the user can supply
+the function as many arguments as provided.
+
+By adding support for variadic functions, we allow users to call user-defined type functions with as many arguments as they wish, providing a more dynamic system.
+
+## Design
+
+The syntax for variadic type functions would be exactly the same as normal variadic functions where `...` can only be legal as the last argument of the function. 
+Utilizing either `table.pack(...)` method or `{...}`, the user would recieve a table of `type`s to use within their code. 
+
+### Examples
+
+Creating a variadic type function:
+
+```luau
+type function f(...)
+    local packed = {...} -- {type}
+    for _, type in packed do
+        -- Use the provided types.
+    end
+end
+
+type a = f<string, number>
+```
+
+Calling a variadic type function from a type alias:
+
+```luau
+type function f(...)
+    local packed = {...}
+    local firstArg = packed[1] -- tag: string
+    local secondArg = packed[2] -- tag: number
+    local thirdArg = packed[3] -- tag: table
+end
+
+type a<T, U, V> = f<T, U, V>
+type b = a<string, number, {}>
+```
+
+## Drawbacks
+
+It's unclear if this proposal has any drawbacks.
+
+## Alternatives
+
+Do nothing, and force type functions to only take pre-defined set of parameters.