patch 9.1.1140: filetype: m17ndb files are not detected

patch 9.1.1140: filetype: m17ndb files are not detected

Problem:  filetype: m17ndb files are not detected
Solution: detect m17ndb files as m17ndb filetype,
          include filetype, syntax and indent files for the
          new filetype (David Mandelberg).

References:

https://www.nongnu.org/m17n/manual-en/m17nDBFormat.html describes the
format. https://git.savannah.nongnu.org/cgit/m17n/m17n-db.git/tree/ has
examples of the files.

closes: #16696

Signed-off-by: David Mandelberg <david@mandelberg.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>