Implement W605, invalid escape sequence

[andersk]

Python 3.6 deprecated unrecognized backslash escape sequences in string literals such as "\c", and they will eventually become errors. Pycodestyle raises W605 invalid escape sequence '\c'. These aren’t adjusted by Black, so it’d be good for Ruff to have this rule too. This probably requires a CST parser (#286).

$ cat a.py
"\c"
$ black a.py
All done! ✨ 🍰 ✨
1 file left unchanged.
$ flake8 a.py
a.py:1:2: W605 invalid escape sequence '\c'
$ python -Wall a.py
/tmp/a.py:1: DeprecationWarning: invalid escape sequence '\c'
  "\c"
$ python -Werror a.py
  File "/tmp/a.py", line 1
    "\c"
    ^^^^
SyntaxError: invalid escape sequence '\c'
$ ruff a.py
No pyproject.toml found.
Falling back to default configuration...

[charliermarsh]

I can implement this via token iteration for now.