Adjust markdown code fences filetype regex
Adjust markdown code fences filetype regex

This highlighter (line 50 of markdown.kak) looks for the filetype
specified by the author at the top of the code fence, e.g.

``` python
print("hello")
```

and highlights the code within using Kakoune's relevant highlighter --
in this case Python.

Some flavours of markdown use curly braces and other characters in the
first line such as the following:

``` {=python}
print("hello")
```

Previously Kakoune recognised `{=python}` but not `{.python}`.  The latter
is Pandoc's flavour of markdown.  This patch adjusts the regex patterns
to recognise the dot notation as well.

lenormf/kakoune,mawww/kakoune,casimir/kakoune,Somasis/kakoune,jkonecny12/kakoune,danr/kakoune,lenormf/kakoune,danr/kakoune,alexherbo2/kakoune,Somasis/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,casimir/kakoune,casimir/kakoune,mawww/kakoune,casimir/kakoune,mawww/kakoune,Somasis/kakoune,lenormf/kakoune,alexherbo2/kakoune,alexherbo2/kakoune,alexherbo2/kakoune,mawww/kakoune,lenormf/kakoune,Somasis/kakoune,danr/kakoune,danr/kakoune,jkonecny12/kakoune