markdown.kak: Clean up code-block and code-span formatting.

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.

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