patch 9.2.0826: highlighting for broken terminals can be improved

patch 9.2.0826: highlighting for broken terminals can be improved

Problem:  highlighting for broken terminals can be improved
          (Maxim Kim)
Solution: Make the highlighting work better when the terminal does not
          support colors (Maxim Kim)

In a colorless or broken terminal emulator current syntax highlighting
is too intense with using bold and underline for various syntax
elements.

In this scenario, when the colors are not available at all, the most
important part of the syntax is the comments, something that is not
executed or compiled.

- Comments are highlighted as bold
- All other syntax elements are set to NONE
- LineNr is not highlighted (was underline)
- SpecialKey is not highlighted (was bold)

fixes:  #20712
closes: #20799

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>