What to do with nqp-mode?

[zakame]

Continuing discussion from melpa/melpa#4984, let me suggest a couple things regarding how do deal with it, in increasing difficulty:

• Remove nqp-mode as it currently stands, being a duplicate of perl6-mode. Mode detection for *.nqp files is done in perl6-detect.el, which means that even current perl6-mode users are not using nqp-mode by default (they'll have to override that in their own auto-mode-alist configs.)
• Make nqp-mode a subset of perl6-mode (a parallel to NQP being a subset of Perl 6,) having the latter mode defined using define-derived-mode. Currently perl6-mode itself (as well as nqp-mode, being a duplicate) derives from prog-mode, so if we choose to keep nqp-mode for later development, then we can keep the nqp- and perl6- bits in separate files.

What do you think?