Default Vi cursor shapes for insert/replace mode

Let's provide a sensible default here. Use a line for "insert" and an
underline for "replace_one" mode.  Neovim does the same, it feels pretty
slick.

As mentioned in #10806

CHANGELOG.rst

@@ -187,6 +187,7 @@ New or improved bindings
   - ``forward-token``, ``backward-token``, ``kill-token``, and ``backward-kill-token`` are similar to the ``*-bigword`` variants but for the whole argument token which includes escaped spaces (:issue:`2014`).
 - The ``accept-autosuggestion`` special input function now returns false when there was nothing to accept.
 - Vi mode has seen some improvements but continues to suffer from the lack of people working on it.
+  - New default cursor shapes for insert and replace mode.
   - Insert-mode :kbd:`ctrl-n` accepts autosuggestions (:issue:`10339`).
   - Outside insert mode, the cursor will no longer be placed beyond the last character on the commandline.
   - When the cursor is at the end of the commandline, a single :kbd:`l` will accept an autosuggestion (:issue:`10286`).

share/functions/fish_vi_cursor.fish

@@ -9,7 +9,15 @@ function fish_vi_cursor -d 'Set cursor shape for different vi modes'
 
     function __fish_vi_cursor --argument-names varname
         if not set -q $varname
-            set varname fish_cursor_unknown
+            switch $varname
+                case fish_cursor_insert
+                    __fish_cursor_xterm line
+                case fish_cursor_replace_one fish_cursor_replace
+                    __fish_cursor_xterm underscore
+                case '*'
+                    __fish_cursor_xterm $fish_cursor_unknown
+            end
+            return
         end
         __fish_cursor_xterm $$varname
     end