Call "fish_command_not_found" if a command wasn't found

Previously, when a command wasn't found, fish would emit the
"fish_command_not_found" *event*.

This was annoying as it was hard to override (the code ended up
checking for a function called `__fish_command_not_found_handler`
anyway!), the setup was ugly,
and it's useless - there is no use case for multiple command-not-found handlers.

Instead, let's just call a function `fish_command_not_found` if it
exists, or print the default message otherwise.

The event is completely removed, but because a missing event is not an error
(MEISNAE in C++-speak) this isn't an issue.

Note that, for backwards-compatibility, we still keep the default
handler function around even tho the new one is hard-coded in C++.

Also, if we detect a previous handler, the new handler just calls it.

This way, the backwards-compatible way to install a custom handler is:

```fish
function __fish_command_not_found_handler --on-event fish_command_not_found
    # do a little dance, make a little love, get down tonight
end
```

and the new hotness is

```fish
function fish_command_not_found
    # do the thing
end
```

Fixes #7293.

doc_src/cmds/fish_command_not_found.rst

@@ -0,0 +1,76 @@
+.. _cmd-fish_cmd_not_found:
+
+fish_command_not_found - what to do when a command wasn't found
+===============================================================
+
+Synopsis
+--------
+
+::
+
+  function fish_command_not_found
+      ...
+  end
+
+
+Description
+-----------
+
+When fish tries to execute a command and can't find it, it invokes this function.
+
+It can print a message to tell you about it, and it often also checks for a missing package that would include the command.
+
+Fish ships multiple handlers for various operating systems and chooses from them when this function is loaded,
+or you can define your own.
+
+It receives the full commandline as one argument per token, so $argv[1] contains the missing command.
+
+When you leave ``fish_command_not_found`` undefined (e.g. by adding an empty function file) or explicitly call ``__fish_default_command_not_found_handler``, fish will just print a simple error.
+
+Example
+-------
+
+A simple handler:
+
+::
+
+    function fish_command_not_found
+        echo Did not find command $argv[1]
+    end
+
+    > flounder
+    Did not find command flounder
+
+Or the handler for OpenSUSE's command-not-found::
+
+    function fish_command_not_found
+        /usr/bin/command-not-found $argv[1]
+    end
+
+Or the simple default handler::
+
+    function fish_command_not_found
+        __fish_default_command_not_found_handler $argv
+    end
+
+Backwards compatibility
+-----------------------
+
+This command was introduced in fish 3.2.0. Previous versions of fish used the "fish_command_not_found" :ref:`event <event>` instead.
+
+To define a handler that works in older versions of fish as well, define it the old way::
+
+  function __fish_command_not_found_handler --on-event fish_command_not_found
+       echo COMMAND WAS NOT FOUND MY FRIEND $argv[1]
+  end
+
+in which case fish will define a ``fish_command_not_found`` that calls it,
+or define a wrapper::
+
+  function fish_command_not_found
+       echo "G'day mate, could not find your command: $argv"
+  end
+
+  function __fish_command_not_found_handler --on-event fish_command_not_found
+       fish_command_not_found $argv
+  end