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

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- [X] There are tests for these changes OR

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tests/unit/servo_config/opts.rs

@@ -2,7 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use servo_config::opts::parse_url_or_filename;
+use servo_config::opts::{parse_url_or_filename, parse_pref_from_command_line};
+use servo_config::prefs::{PrefValue, PREFS};
 use std::path::Path;
 
 #[cfg(not(target_os = "windows"))]
@@ -67,3 +68,26 @@ fn test_argument_parsing_special() {
     assert_eq!(url.query(), None);
     assert_eq!(url.fragment(), None);
 }
+
+#[test]
+fn test_parse_pref_from_command_line() {
+    // Test with boolean values.
+    parse_pref_from_command_line("testtrue=true");
+    assert_eq!(*PREFS.get("testtrue"), PrefValue::Boolean(true));
+    parse_pref_from_command_line("testfalse=false");
+    assert_eq!(*PREFS.get("testfalse"), PrefValue::Boolean(false));
+
+    // Test with numbers.
+    parse_pref_from_command_line("testint=42");
+    assert_eq!(*PREFS.get("testint"), PrefValue::Number(42 as f64));
+    parse_pref_from_command_line("testfloat=4.2");
+    assert_eq!(*PREFS.get("testfloat"), PrefValue::Number(4.2));
+
+    // Test default (string).
+    parse_pref_from_command_line("teststr=str");
+    assert_eq!(*PREFS.get("teststr"), PrefValue::String("str".to_owned()));
+
+    // Test with no value.
+    parse_pref_from_command_line("testempty");
+    assert_eq!(*PREFS.get("testempty"), PrefValue::Boolean(true));
+}