Non-idiomatic use of Ruby's Regexp.new where String start_with? will do

[mathias]

As part of doing some Profiling, I noticed Primer::Classify::Utilities | infer_selector_key showing up and happened to take a look at its implementation. There's a Hash of about 12 Regexps getting instantiated that don't seem to be needed, and these get matched enough in various usage that even a small speed up might be worth it, if that method gets called thousands of times.

Generally in Ruby, things like String's built-in methods will be faster because they're written in C, unless we need features of Regexp like capture groups. I've got a PR that will fast-follow this Issue getting created, that proposes using Strings in place of Regexp.new.