Support 'R' prefix for raw string literals

[bmeisels]

Following the discussion on adafruit/Adafruit_CircuitPython_IL0373#7

Circuitpython currently supports raw string / byte literals with the 'r' prefix but doesn't support the 'R' prefix.

This should be changed to support compatibility with CPython.

"Both string and bytes literals may optionally be prefixed with a letter 'r' or 'R'; such strings are called raw strings and treat backslashes as literal characters." (excerpt from https://docs.python.org/3.7/reference/lexical_analysis.html)

[dhalbert]

Upper or lower case may be used for f, u, r, or b prefixes, so if we support one, maybe we should support all. https://docs.python.org/3.8/reference/lexical_analysis.html#string-and-bytes-literals

stringliteral   ::=  [stringprefix](shortstring | longstring)
stringprefix    ::=  "r" | "u" | "R" | "U" | "f" | "F"
                     | "fr" | "Fr" | "fR" | "FR" | "rf" | "rF" | "Rf" | "RF"
shortstring     ::=  "'" shortstringitem* "'" | '"' shortstringitem* '"'
longstring      ::=  "'''" longstringitem* "'''" | '"""' longstringitem* '"""'
shortstringitem ::=  shortstringchar | stringescapeseq
longstringitem  ::=  longstringchar | stringescapeseq
shortstringchar ::=  <any source character except "\" or newline or the quote>
longstringchar  ::=  <any source character except "\">
stringescapeseq ::=  "\" <any source character>
bytesliteral   ::=  bytesprefix(shortbytes | longbytes)
bytesprefix    ::=  "b" | "B" | "br" | "Br" | "bR" | "BR" | "rb" | "rB" | "Rb" | "RB"
shortbytes     ::=  "'" shortbytesitem* "'" | '"' shortbytesitem* '"'
longbytes      ::=  "'''" longbytesitem* "'''" | '"""' longbytesitem* '"""'
shortbytesitem ::=  shortbyteschar | bytesescapeseq
longbytesitem  ::=  longbyteschar | bytesescapeseq
shortbyteschar ::=  <any ASCII character except "\" or newline or the quote>
longbyteschar  ::=  <any ASCII character except "\">
bytesescapeseq ::=  "\" <any ASCII character>