rc markdown: Highlight trailing spaces properly

This commit addresses the following issues:

* highlight trailing space characters with the `meta` face, instead of
  `PrimarySelection`
* make the regex more readable by using a capture group in stead of
  `\K`
* specifically match space characters, not other horizontal whitespace
  characters
* match two or more space characters

Reference[1]:

> When you do want to insert a <br /> break tag using Markdown, you
> end a line with two or more spaces, then type return.

[1] https://daringfireball.net/projects/markdown/syntax#p

Note that the original reproducer doesn't seem to work anymore,
probably because of changes made to how lists are highlighted.

Fixes #911

0 files changed, 0 insertions, 0 deletions