Default the PathConverter (and descendants) to be non part isolating

This is likely to be the expected for anything that is or extends the
PathConverter and by adding it explicitly I hope it will cause less
confusion, as seen in a recent issue 2783.

Note the PathConverter was already non part isolating due to the
BaseConverter check.

CHANGES.rst

@@ -12,6 +12,7 @@ Unreleased
 -   Add the ``"werkzeug.profiler"`` item to the  WSGI ``environ`` dictionary
     passed to `ProfilerMiddleware`'s `filename_format` function. It contains
     the ``elapsed`` and ``time`` values for the profiled request. :issue:`2775`
+-   Explicitly marked the PathConverter as non path isolating. :pr:`2784`
 
 
 Version 2.3.8

src/werkzeug/routing/converters.py

@@ -119,6 +119,7 @@ class PathConverter(BaseConverter):
     :param map: the :class:`Map`.
     """
 
+    part_isolating = False
     regex = "[^/].*?"
     weight = 200