Remove local rewrite flag and enable it by default

[alanechang]

This PR removes the --rewrite-old-style-jane-street-local-annotations flag and enables rewriting with the new syntax by default.

A few consequences of this change are:

• --erase-jane-syntax now erases the old style local annotation too. See local-erased.ml.ref for differences
• In order for the AST equality checks to pass when we are modifying the tree deliberately, a new ignore_local_annot_differences option is added to ignore changes around local annotations in the standard AST. This option is set automatically when the legacy syntax is encountered. The alternative here is to convert old local syntax to the new or vice versa during normalization, which is non-trivial and will involve duplicating logic from jane syntax. The standard AST normalization is changed to convert old local syntax into the new syntax in order for the equality check to pass.

[alanechang]

Removed the logic for erasing legacy local annotation syntax in Fmt_ast.ml and added a note about it in the test:

ocamlformat/test/passing/tests/local_erased.ml

Lines 1 to 5 in eceb638

	(* Note that this file cannot contain legacy local annotation syntax. If a
	file with legacy local annotations wants to have its syntax erased, call
	ocamlformat once without the --erase-jane-syntax flag to rewrite it into
	the new syntax and then call ocamlformat a second time to erase the
	syntax. *)

[alanechang]

@goldfirere Added one more commit to handle the pattern case differently. This fixes an issue caught by the CI tests.

PR should be ready to merge if that commit looks good.

[goldfirere]

Took a look at that last commit. Looks fine. Let's merge.