Fixes #205328

after attempting to match a language alias against the whole line (and failing)
it will fall back to attempting to match only the initial (whitespace separated) language name

the intention for this is for my extension
json textmate is a subset of normal json
I wanted to put a codeblock in displaying some json textmate code
but if my extension isn't installed, I wanted it to default back to normal json
which is what Github and the builtin Markdown extension does
but the extension README.md preview does not

lang.split(/\s+|:|,|(?!^)\{|\?]/, 1)[0]
splits the language line once on one of the following symbols :,{?
then returns the first section
(leading whitespace is already stripped from the lang line)