Leading underscore in APL variable names (GNU, Dyalog)

[xpqz]

Two small tweaks to the APL lexer

1. Add all APL file suffixes
2. Allow leading _ in variable names (GNU APL, Dyalog)

[rodrigogiraoserrao]

This PR is more useful than it may seem, as _← is a common idiom in APL programming.
There's also the fact that leading underscores are used commonly as the leading character of dop names.

[Anteru]

Thanks! Can you please regenerate the mapfiles as well (make mapfiles), otherwise the file endings won't get picked up. Would you mind adding/extending one of the APL tests to capture this as well? Not required but would be nice to have so it doesn't regress.

[xpqz]

@Anteru Thanks for your comments. I've regenerated the mapfiles and added a testcase (there weren't many for APL...) to demonstrate the modification as per your suggestions.

[Anteru]

Merged, thanks a lot!