unify matcher name restrictions and clearer error on bad param/matcher names

[gtm-nayan]

closes #5261

There was a discrepancy between the allowed names for matcher files and matchers in route names, took the opportunity to remove that as well. Was it there for a special reason?

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[Rich-Harris]

thanks, not sure if there was a reason for the discrepancy. it looks like we wanted to disallow names that began with a digit, but i'm not sure why that restriction would be necessary

[Conduitry]

Names not beginning with a digit could have been from when we wanted the matcher name to be the name of a named export? I don't really remember. I agree that there's not really a reason to restrict that, since it's just the name of a file.