Add TYPES documentation

specs/TYPES.md

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+# Types
+C2 has both built-in types and user-defined types.
+
+## Builtin types
+C2 has the following types builtin:
+ - `void`
+ - `i8`
+ - `i16`
+ - `i32`
+ - `i64`
+ - `u8`
+ - `u16`
+ - `u32`
+ - `u64`
+
+## Type Aliases
+Types can be aliased to different names using the alias keyword.
+Here's a list of the default builtin aliases.
+
+```c2
+alias int = i32;
+alias uint = u32;
+alias char = u8;
+alias string = char[];
+```