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.
