feat: make generic.is_url() accept dollar-quoted userinfo fragments

Part of supporting authenticated remote styles requires a way to specify
authinfo without necessarily hardcoding a sensitive token in the config
file.

This is done by interpolating an environment variable if the url
username fragment begins with a `$`, but that fails the original
`nitpick.generic.URL_RE` pattern, because `\w` weirdly doesn't match
`$`.

src/nitpick/generic.py

@@ -15,7 +15,7 @@
 from nitpick.constants import DOUBLE_QUOTE, PROJECT_NAME, SEPARATOR_FLATTEN, SEPARATOR_QUOTED_SPLIT, SINGLE_QUOTE
 from nitpick.typedefs import JsonDict, PathOrStr
 
-URL_RE = re.compile(r"[a-z]+://\w+")
+URL_RE = re.compile(r"[a-z]+://[\$\w]\w*")
 
 
 def get_subclasses(cls):
@@ -222,6 +222,12 @@ def is_url(url: str) -> bool:
     True
     >>> is_url("github://andreoliwa/nitpick/styles/black")
     True
+    >>> is_url("github://$VARNAME@andreoliwa/nitpick/styles/black")
+    True
+    >>> is_url("github://LitErAl@andreoliwa/nitpick/styles/black")
+    True
+    >>> is_url("github://notfirst$@andreoliwa/nitpick/styles/black")
+    True
     """
     return bool(URL_RE.match(url))