py/lexer: Support raw f-strings.

Support for raw str/bytes already exists, and extending that to raw
f-strings is easy.  It also reduces code size because it eliminates an
error message.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/string_fstring.py

@@ -75,3 +75,7 @@ print(
     f"cd---------------------------------"
     f"e{y}f---------------------------------"
 )
+
+# Raw f-strings.
+print(rf"\r\a\w {'f'} \s\t\r\i\n\g")
+print(fr"\r{x}")