helix: Apply modification (e.g. switch case) on a single character only in helix mode (#38119)

Closes #34192

Without selection, only current character would be affected.

Also if #38117 is merged too, then transformations in SelectMode behave
correctly too and selection is not collapsed.

Release Notes:

- helix: Implemented `~`, `` ` ``, `` Alt-` `` correctly in normal and
select modes

---------

Co-authored-by: Jakub Konka <kubkon@jakubkonka.com>

assets/keymaps/vim.json

@@ -442,9 +442,8 @@
       ">": "vim::Indent",
       "<": "vim::Outdent",
       "=": "vim::AutoIndent",
-      "g u": "vim::PushLowercase",
-      "g shift-u": "vim::PushUppercase",
-      "g ~": "vim::PushOppositeCase",
+      "`": "vim::ConvertToLowerCase",
+      "alt-`": "vim::ConvertToUpperCase",
       "g q": "vim::PushRewrap",
       "g w": "vim::PushRewrap",
       "insert": "vim::InsertBefore",