markup: change name of assert_valid_name() so that it's clear it's forever

D programmers would assume anything called assert_x would go away if
the code is compiled in release mode.  That isn't the case here.

Signed-off-by: Peter Williams <pwil3058@gmail.com>
markup: move AbstractMarkupParser() to dguitk/utils/markup.d

Signed-off-by: Peter Williams <pwil3058@gmail.com>
markup: simplify access to characters in current text

Signed-off-by: Peter Williams <pwil3058@gmail.com>