parser: fn type declaration does not check already registered name

[felipensp]

Fix #20730

@[translated]
module vnkxcb

type Nk_size = u32

union Nk_handle {
	ptr voidptr
	id int
}

enum Nk_plugin_alloc {
	nk_tree_node
	nk_tree_tab
}

type Nk_plugin_alloc = fn (Nk_handle, voidptr, Nk_size) voidptr // already registered name as Enum below

[spytheman]

Excellent work.

[odiroot]

That was super fast, thank you :)