keymap: Add ability to update user keymaps (#33487)

Closes #ISSUE

The ability to update user keybindings in their keymap is required for
#32436. This PR adds the ability to do so, reusing much of the existing
infrastructure for updating settings JSON files.

However, the existing JSON update functionality was intended to work
only with objects, therefore, this PR simply wraps the object updating
code with non-general keymap-specific array updating logic, that only
works for top-level arrays and can only append or update entries in said
top-level arrays. This limited API is reflected in the limited
operations that the new `update_keymap` method on `KeymapFile` can take
as arguments.

Additionally, this PR pulls out the existing JSON updating code into its
own module (where array updating code has been added) and adds a
significant number of tests (hence the high line count in the diff)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

Cargo.lock

@@ -14554,12 +14554,12 @@ dependencies = [
  "serde_json",
  "serde_json_lenient",
  "smallvec",
- "streaming-iterator",
  "tree-sitter",
  "tree-sitter-json",
  "unindent",
  "util",
  "workspace-hack",
+ "zlog",
 ]
 
 [[package]]