Open command script in external editor on Alt+o

Fish functions are great for configuring fish but they don't integrate
seamlessly with the rest of the system. For tasks that can run outside fish,
writing scripts is the natural approach.

To edit my scripts I frequently run

    $EDITOR (which my-script)

Would be great to reduce the amount typing for this common case (the names
of editor and scripts are usually short, so that's a lot of typing spent on
the boring part).

Our Alt+o binding opens the file at the cursor in a pager.  When the cursor
is in command position, it doesn't do anything (unless the command is actually
a valid file path). Let's make it open the resolved file path in an editor.

In future, we should teach this binding to delegate to "funced" upon seeing
a function instead of a script. I didn't do it yet because funced prints
messages, so it will mess with the commandline rendering if used from
a binding.  (The fact that funced encourages overwriting functions that
ship with fish is worrysome. Also I'm not sure why funced doesn't open the
function's source file directly (if not sourced from stdin). Persisting the
function should probably be the default.)

Alternative approach: I think other shells expand "=my-script" to
"/path/to/my-script".  That is certainly an option -- if we do that we'd want
to teach fish to complete command names after "=".  Since I don't remember
scenarios where I care about the full path of a script beyond opening it in
my editor, I didn't look further into this.

Closes #10266

doc_src/interactive.rst

@@ -331,7 +331,7 @@ Some bindings are common across Emacs and vi mode, because they aren't text edit
 
 - :kbd:`Alt`\ +\ :kbd:`L` lists the contents of the current directory, unless the cursor is over a directory argument, in which case the contents of that directory will be listed.
 
-- :kbd:`Alt`\ +\ :kbd:`O` opens the file at the cursor in a pager.
+- :kbd:`Alt`\ +\ :kbd:`O` opens the file at the cursor in a pager. If the cursor is in command position and the command is a script, it will instead open that script in your editor. The editor is chosen from the first available of the ``$VISUAL`` or ``$EDITOR`` variables.
 
 - :kbd:`Alt`\ +\ :kbd:`P` adds the string ``&| less;`` to the end of the job under the cursor. The result is that the output of the command will be paged.