Don't highlight # in interior of heading

[jez]

The regex is a bit gnarly, so to break it down, the old regex has two
alternatives:

• ^#\{1,6}

  Matches 1–6 opening hashes at the start of the line.

• \\\@<!#\+\(\s*.*$\)\@=

  Matches one or more unescaped hashes when followed by \s*.*$

  It's trying to match the ## at the end of a line, to allow
  highlighting code like this:

  ## Some heading ##

  The problem is that this second condition is effectively always true
  (to see, consider that the regex engine could choose to apply \s
  zero times, then .* until the end of the line).

  That bug causes something like vim-pandoc/vim-pandoc-syntax#404 in
  an ATX heading to treat the # as an additional heading marker.

The new regex fixes that by ensuring that there's at least one space
character before the #, and ensuring that the match highlighting does
not include that required space character (with \zs).

So the full replacement is now:

\(^#\{1,6}\|\s\zs\\\@<!#\+\s*$\)

And its second alternative works like this:

• \s — whitespace before the closing hashes
• \zs — begin the highlighted match here, excluding that whitespace
• \\\@<! — the hash is not escaped
• #\+ — one or more hashes
• \s*$ — only optional whitespace may follow before the line ends

The test fixture adds an example, but to my knowledge these are just
used for visual inspection, there's nothing asserting these tests.

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