api: Flatten and simplify Servo preferences (#34966)

Flatten and simplify Servo's preferences code. In addition, have both
preferences and options passed in as arguments to `Servo::new()` and
make sure not to use the globally set preferences in `servoshell` (as
much as possible now).

Instead of a complex procedural macro to generate preferences, just
expose a very simple derive macro that adds string based getters and
setters.

- All command-line parsing is moved to servoshell.
- There is no longer the concept of a missing preference.
- Preferences no longer have to be part of the resources bundle because
  they now have reasonable default values.
- servoshell specific preferences are no longer part of the preferences
  exposed by the Servo API.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/bindings/codegen/run.py

@@ -89,10 +89,31 @@ def generate(config, name, filename):
 
 
 def add_css_properties_attributes(css_properties_json, parser):
+    def map_preference_name(preference_name: str):
+        """Map between Stylo preference names and Servo preference names as the
+        `css-properties.json` file is generated by Stylo. This should be kept in sync with the
+        preference mapping done in `components/servo_config/prefs.rs`, which handles the runtime version of
+        these preferences."""
+        MAPPING = [
+            ["layout.unimplemented", "layout_unimplemented"],
+            ["layout.threads", "layout_threads"],
+            ["layout.legacy_layout", "layout_legacy_layout"],
+            ["layout.flexbox.enabled", "layout_flexbox_enabled"],
+            ["layout.columns.enabled", "layout_columns_enabled"],
+            ["layout.grid.enabled", "layout_grid_enabled"],
+            ["layout.css.transition-behavior.enabled", "layout_css_transition_behavior_enabled"],
+            ["layout.writing-mode.enabled", "layout_writing_mode_enabled"],
+            ["layout.container-queries.enabled", "layout_container_queries_enabled"],
+        ]
+        for mapping in MAPPING:
+            if mapping[0] == preference_name:
+                return mapping[1]
+        return preference_name
+
     css_properties = json.load(open(css_properties_json, "rb"))
     idl = "partial interface CSSStyleDeclaration {\n%s\n};\n" % "\n".join(
         "  [%sCEReactions, SetterThrows] attribute [LegacyNullToEmptyString] DOMString %s;" % (
-            ('Pref="%s", ' % data["pref"] if data["pref"] else ""),
+            (f'Pref="{map_preference_name(data["pref"])}", ' if data["pref"] else ""),
             attribute_name
         )
         for (kind, properties_list) in sorted(css_properties.items())