feat(gs/achilles): Prefix top-level ascriptions with ty

This makes parsing less ambiguous, which is nice (we can continue to not
actually care about indentation!) and aligns nicely with `fn` for the
declaration itself.

Change-Id: Id48f064e2a1e01c5105297be355d0991b312b76d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2615
Tested-by: BuildkiteCI
Reviewed-by: glittershark <grfn@gws.fyi>

users/glittershark/achilles/ach/functions.ach

@@ -1,3 +1,8 @@
+ty id : fn a -> a
 fn id x = x
+
+ty plus : fn int -> int
 fn plus (x: int) (y: int) = x + y
+
+ty main : fn -> int
 fn main = plus (id 2) 7