fixed quirks::parse_pattern of WPT [{ "pathname": "/(\\m)" }]

[edgul]

URLPattern crate doesn't seem to test for quirks::parse_pattern but one of the failing tests when invoked from firefox

{
    "pattern": [{ "pathname": "/(\\m)" }],
    "expected_obj": "error"
  },

Relevant spec location: https://urlpattern.spec.whatwg.org/#compile-a-component
I'm not seeing the place in the algorithm where we should explicitly run the parser eval, but the spec notes the following:
"
The specification uses regular expressions to perform all matching, but this is not mandated. Implementations are free to perform matching directly against the part list when possible; e.g. when there are no custom regexp matching groups. If there are custom regular expressions, however, its important that they be immediately evaluated in the compile a component algorithm so an error can be thrown if they are invalid.
"

This is a bit of a hack and probably we should just be feeding Spidermonkey regex into the pattern, but it seems to fix the test without breaking anything else.

[edgul]

Hey @lucacasonato, I'm not able to add a reviewer, can you take a look?