Use regions to avoid highlighting the commit message as a diff.
The new method will fail if one line of the commit message matches
'^diff --git' but that is fairly unlikely.
Fixes#2371
Previously, one of the syntaxes for italic was (greatly summarized) something
like this:
[^_]_[^_]+_[^_]
That is to say, the regex matched the blanks on both sides of the italic span,
as well as the actual span content. That means that if you had consecutive
italic words:
_some_ _italic_ _words_
...only the odd-numbered words would be highlighted: the space after "_some_"
was counted as part of that span, so it wasn't available as part of "_italic_"
and therefore "_italic_" wouldn't be highlighted. Likewise, if the first word
in a buffer was italic, it wouldn't be highlighted because the first underscore
was not preceded by a non-underscore character!
Now we use lookahead/lookbehind assertions, which don't count as part of the
matched span, so consecutive spans don't interfere with one another.
Fixes#2111.
Previously, a code block was anything between triple-backtics, including inline
blocks:
some text ```
not a codeblock, but highlighted as one
``` other text
and even if the closing backticks had the wrong indent:
```
this is a code block containing a triple backtick
```
this is still a code block, but Kakoune thinks otherwise
```
Now we use the -match-capture flag to ensure the start and end fences have
exactly the same indent.
Previously, the generic code-block region was defined first, which meant that
it took priority over all the language-specific highlighters. Now we define
the generic code-block highlighting *after* the others, which fixes#2304.
Previously, code-spans were defined as ordinary inline markup, but in Markdown
ordinary formatting doesn't work inside code-spans. Therefore, they are now
regions unto themselves, defined according to section 6.3 of the CommonMark
spec <https://spec.commonmark.org/0.28/#code-spans>, which addresses a comment
on #2111.
