Auto merge of #14863 - charlesvdv:prefs, r=jdm

Allow cli prefs to have numerical value

I'm not sure as I'm new with servo but shouldn't the new function ```parse_opt_prefs``` be in ```prefs.rs``` instead of ```opts.rs``` ?

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- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14842

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components/config/opts.rs

@@ -889,14 +889,7 @@ pub fn from_cmdline_args(args: &[String]) -> ArgumentParsingResult {
     prefs::add_user_prefs();
 
     for pref in opt_match.opt_strs("pref").iter() {
-        let split: Vec<&str> = pref.splitn(2, '=').collect();
-        let pref_name = split[0];
-        let value = split.get(1);
-        match value {
-            Some(&"false") => PREFS.set(pref_name, PrefValue::Boolean(false)),
-            Some(&"true") | None => PREFS.set(pref_name, PrefValue::Boolean(true)),
-            _ => PREFS.set(pref_name, PrefValue::String(value.unwrap().to_string()))
-        };
+        parse_pref_from_command_line(pref);
     }
 
     if let Some(layout_threads) = layout_threads {
@@ -946,6 +939,20 @@ pub fn set_defaults(opts: Opts) {
     }
 }
 
+pub fn parse_pref_from_command_line(pref: &str) {
+    let split: Vec<&str> = pref.splitn(2, '=').collect();
+    let pref_name = split[0];
+    let value = split.get(1);
+    match value {
+        Some(&"false") => PREFS.set(pref_name, PrefValue::Boolean(false)),
+        Some(&"true") | None => PREFS.set(pref_name, PrefValue::Boolean(true)),
+        Some(value) => match value.parse::<f64>() {
+            Ok(v) => PREFS.set(pref_name, PrefValue::Number(v)),
+            Err(_) => PREFS.set(pref_name, PrefValue::String(value.to_string()))
+        }
+    };
+}
+
 #[inline]
 pub fn get() -> &'static Opts {
     &OPTIONS