When expanding the `let` pattern, one would end up with something like "let I: I${4:type} = I;" (I representing the cursor jump marks), so the nested jump marks are inserted literally.
I fixed that unwanted behaviour by replacing the `let` and `letm` patterns by `let` (for immutable variable bindings with type inference), `lett` (for immutable variable bindings with explicit type annotation), `letm` (for mutable variable bindings with type inference) and `lettm` (for mutable variable bindings with explicit type annotation).
