Add raw string tokens

[ManuelHentschel]

What problem did you solve?

Adds support for raw strings introduced in R 4.0.0. For details, see ? Quotes

How can I check this pull request?

E.g. the following expressions are valid raw strings in R 4.x.x:

r'(asdf\qwer)'
R"-[test]-"
r'--{\/'\/}--'

producing the output

> r'(asdf\qwer)'
[1] "asdf\\qwer"
> R"-[test]-"
[1] "test"
> r'--{\/'\/}--'
[1] "\\/'\\/"

With this PR, they should be highlighted correctly in the editor.

Note: I'm not very familiar with textmate grammars, so if this can be implemented more efficiently or should use different names etc., feel free to modify.

[renkun-ken]

Thanks for working on this, closing #287.

[renkun-ken]

Works nicely.