Support custom typespec

[gyson]

• Support custom typespec in type.exs conf file. (kind of like type definitions from Typescript)
• The scope of this custom typespec should be limited to the package.
• It would override exist typespec.

example type.exs file:

[
  typespec: %{
    Enum => quote do 
      @type some_special_type :: any()

      @spec map(T.p(Enumerable, x), (x -> y)) :: [y] when x: any(), y: any()
    end
  }
]