RFC: type externname - Method to get an ExternType's name

docs/udtf-type-extern-name.md

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+# type ``externname`` method
+
+## Summary
+
+Add a new method to ``type`` for User Defined Type Functions, ``type.externname``. Which will retrieve a ``ExternType``'s name.
+
+## Motivation
+
+**Use cases:**
+- A lazy _(but therefore also quick way?)_ to filter out extern types (e.g. from ``declare extern type``)
+  - e.g. external based types that are not accessible through ``types.`` such as ``vector`` for instance
+  - embedders such as Roblox, may also have external based types that can't be filtered without passing a type into the function.
+- Useful for debug purposes when using ``print`` within a type function.
+
+
+Currently, you can only do this, or other tricks:
+
+```lua
+--!strict
+type function isType(input, whatToCheck)
+  return (input == whatToCheck)
+end
+
+type function vectorOnly(input, whatToCheck)
+  local matches = isType(input, whatToCheck)
+  if (matches) then
+    print(matches, "It is a vector type")
+    return input
+  end
+
+  return types.unknown
+end
+
+type a = vectorOnly<typeof(vector.zero), vector>
+```
+
+
+**What it would solve:**
+- You don't have to pass in a sample of a type that you want to check
+- Drawback: If two ExternTypes are ever named the same, it would be an inaccurate check, hence why above it mentions _"lazy"_
+
+
+## Design
+
+This is the bulk of the proposal. Explain the design in enough detail for somebody familiar with the language to understand, and include examples of how the feature is used.
+
+I tried an implementation:
+- https://github.com/karl-police/luau/commit/26b66c79d4d68e5f18601f619082b4d9b62473aa
+- https://github.com/karl-police/luau/commit/03bef7130a9cdbf550b6e0bbcdaf5c9a562ef427
+
+This is its **Unit Test**.
+
+```lua
+declare extern type CustomExternType with
+    function testFunc(self): number
+end
+
+type function pass(arg, compare)
+    if (arg:is("extern")) then
+        assert(arg:externname() == compare:value())
+    end
+
+    return types.unknown
+end
+
+type a = pass<CustomExternType, "CustomExternType">
+type b = pass<vector, "vector">
+```
+
+
+### ``type`` changes
+
+| New/Update | Instance Methods | Type | Description |
+| ------------- | ------------- | ------------- | ------------- |
+| New | `externname()` | `string?` | Returns the name of an ExternType or 'nil' if there's no name. |
+
+**OR**
+
+| New/Update | Instance Methods | Type | Description |
+| ------------- | ------------- | ------------- | ------------- |
+| Update | `name()` | `string?` | Returns the name of a generic or ExternType, or 'nil' if there's no name. |
+
+
+
+### Example
+
+```lua
+declare extern type CustomExternType with
+    function testFunc(self): number
+end
+
+type function onlyCustomExternType(input)
+    assert(input:is("extern"))
+
+    if (input:externname() == "CustomExternType") then
+      -- ...
+      return input
+    else
+      error("type is not named CustomExternType")
+    end
+end
+
+type a = onlyCustomExternType<CustomExternType>
+```
+
+
+## Drawbacks
+
+I don't know if a generic can be directly passed into a type function and then use ``type:name()``.
+But Generic Names are being collected.
+
+If ``type:externname()`` would copy the ``externType->name`` into the TypeFunction's ExternType, maybe it would be inefficient for memory, because the name already exists.
+In my first implementation, it doesn't do that though within the "serialize" process, instead it's just "read on request", but without caching.
+
+It is currently confusing that there's ``:value()`` but the function name itself, doesn't speak out _"Hey, I am only for singletons"_ or similar.
+Hence why I am wondering whether if something like this should exist, whether it should be ``:externname()`` or ``:name()``
+
+
+## Alternatives
+
+An alternative would be to have the ability to require "upvalue" or external types into the type function.