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}")

tests/cmdline/cmd_parsetree.py.exp

@@ -1,6 +1,6 @@
 ----------------
 [   1] file_input_2(1) (n=10)
-         tok(6)
+         tok(5)
 [   4]   \(rule\|for_stmt\)(22) (n=4)
            id(i)
 [   4]     \(rule\|atom_paren\)(45) (n=1)
@@ -9,7 +9,7 @@
            NULL
 [   6]   \(rule\|expr_stmt\)(5) (n=2)
            id(a)
-           tok(16)
+           tok(15)
 [   7]   \(rule\|expr_stmt\)(5) (n=2)
            id(b)
            str(str)

tests/cpydiff/core_fstring_raw.py

@@ -1,8 +0,0 @@
-"""
-categories: Core
-description: Raw f-strings are not supported
-cause: MicroPython is optimised for code space.
-workaround: Unknown
-"""
-
-rf"hello"