Add syntax folding to 'raw string' region

[AriSweedler]

I have testcases that need JSON as input. Instead of making a file for each one, I inline the JSON as a raw string. It takes up too much screen real estate.
I do this on my machine, I think other people might appreciate it, too.

[AriSweedler]

I would like to bump this

[AriSweedler]

Hello?

[mattn]

Sorry delay. Could you please provide error case?

[AriSweedler]

It is not an error case. The syntax region around the raw string literal has been written successfully. This PR is a feature request. I have this on my machine, and I think that it is a much saner default than currently exists.

I want a raw string region like this to add a layer of folding

auto my_string = R"delim(
Hello
World
)delim";