Improve string syntax for DB, DM directives (all aliases will automatically work too)

Mpm's DB, DEFB, DM, DEFM directives only accept "" syntax. Improve it to allow strings (multiple bytes) encloses with ''. This will make it much more easy to wrap " character inside a ' ' quoted string, and vice versa.

This will help to parse lots of old assembler sources for 8080, etc, and for many other Z80-related source codes.

See https://gitlab.com/b4works/mpm/-/blob/master/asmtests/directives.asm for examples of different syntax options for DB, DM, etc.