Support for OCaml 5.3 effect syntax

[Zeta611]

As mentioned in #2559, this provides a support for OCaml 5.3 effect syntax.

[Julow]

Sorry for the extremely slow reply. We've been focusing on the 5.2 syntax support and couldn't allocate a lot of time to the rest.
I've fixed the conflicts in Ast.ml and added a test. Let's merge if the CI doesn't catch any bugs.

Huge thanks for your contribution :)

[Zeta611]

No problem! :)

I see few tests have been failed due to build error---is it something to be worried about?

[Julow]

The CI is failing when testing on a larger corpus of code with errors like this:

File "code/infer/infer/src/integration/IssuesTest.ml", line 17, characters 50-56:
17 |   Option.iter taint_policy_privacy_effect ~f:(fun effect ->
                                                       ^^^^^^
Error: Syntax error
File "code/irmin/src/irmin/node_intf.ml", line 127, characters 7-13:
127 |   type effect := expected_depth:int -> node_key -> t option
             ^^^^^^
Error: Syntax error

5.3 also have a mechanism for disabling keywords in the lexer, that would have to be backported first. I'll work on backporting this and making it work with the ocaml-version that ocamlformat has.