Auto merge of #17967 - jdm:parsefun, r=SimonSapin

CSS parsing optimizations

These address some small inefficiencies that showed up while profiling the myspace talos test.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

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ports/geckolib/error_reporter.rs

@@ -77,7 +77,7 @@ fn escape_css_ident(ident: &str) -> String {
         return ident.into()
     }
 
-    let mut escaped = String::new();
+    let mut escaped = String::with_capacity(ident.len());
 
     // A leading dash does not need to be escaped as long as it is not the
     // *only* character in the identifier.