R 4.0 raw string support

[renkun-ken]

Describe the bug

R 4.0 introduces raw string in the form of r"(hello, "world")" (see https://stat.ethz.ch/R-manual/R-devel/library/base/html/Quotes.html). vscode-R does not correctly handle such raw strings in the following aspects:

Suppose we have the following code:

text <- r"(hello, "world")"
dir <- r"(c:\windows\users)"

1. Typing the above code results in an superflous ending quote since r" does not trigger auto-quote but )" triggers another "" in the end.
2. The syntax does not look correct in terms of string and quotes, and escape characters.
3. Sending the code to active terminal results in an auto-quote:

To reproduce, use the edd/r-4.0 PPA introduced in http://dirk.eddelbuettel.com/papers/r4_r400_binaries.pdf.

Environment (please complete the following information):

• OS: Ubuntu 20.04
• VSCode Version: 1.44.2
• R Version: 4.0.0 RC (2020-04-16 r78240)
• vscode-R version: 1.2.8

[gowerc]

@renkun-ken I think this can be closed :)