Change class regex to match elm source

[kdmurthy]

Just a small modification to the regex for use with elm source class method.

[bradlc]

Hey @kdmurthy. Thank you, but I think for now I'd like to keep the built-in support to things that look like HTML class attributes (i.e. they use = to separate the key and value).

You could enable support for elm using a combination of the includeLanguages and experimental.classRegex settings