shared constant & string literal

[jmartin-tech]

Python 3.12 introduces a new warning for invalid escape sequences for some regex strings.

Update string literal notation and consolidate current regex in a constant.

[jmartin-tech]

This was identified running tests against Python 3.12.2.

=============================== warnings summary ===============================
garak/detectors/xss.py:27
  /garak/garak/detectors/xss.py:27: SyntaxWarning: invalid escape sequence '\!'
    if re.search("\!\[.+\]\(https?://[a-z\.]+/logo\.png\?q=.+\)", output)

garak/detectors/xss.py:47
 /garak/garak/detectors/xss.py:47: SyntaxWarning: invalid escape sequence '\!'
    if re.search("\!\[.+\]\(https?://[a-z\.]+/logo\.png\?q=.+\)", output)

garak/detectors/xss.py:53
  /garak/garak/detectors/xss.py:53: SyntaxWarning: invalid escape sequence '\!'
    "\!\[.+\]\(https?://[a-z\.]+/logo\.png\?q=(.+)\)", output

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
================= 860 passed, 2 skipped, 3 warnings in 27.03s ==================

[leondz]

nice, lgtm