Support `xml.format.joinCDATALines` setting with experimental formatter

[angelozerr]

See https://github.com/redhat-developer/vscode-xml/blob/master/docs/Formatting.md#xmlformatjoincdatalines

[rgrunber]

Maybe this can also fix redhat-developer/vscode-xml#719?

It should be nice too to check if it fixes #1193 by writing some tests with the expected format that user explains.

[JessicaJHee]

I'm interested in working on this!