Supports mode annotation on function body

[riaqn]

This PR adds the support for the following syntax:

let foo a b @ modes = ..
let foo a b : ty @@ modes = ..
fun a b @ modes -> ..
fun a b : ty @@ modes -> ..

Please review by commits.

Please do not merge until I finish the internal migration for the new parsetree.

[github-actions]

Parser Change Checklist

This PR modifies the parser. Please check that the following tests are updated:

• parsetree/source_jane_street.ml

This test should have examples of every new bit of syntax you are adding. Feel free to just check the box if your PR does not actually change the syntax (because it is refactoring the parser, say).

[tdelvecchio-jsc]

Liam to do further review since I realized I don't have context on typing/

[liam923]

(I did not review the changes in parser.mly under the premise that @tdelvecchio-jsc already reviewed it.)

I'd like to see a test that shows that this allows a stronger interface to be inferred for a ml-only module.