config: Add a pref for the experimental accessibility code (#42333)

This adds a pref which will be used to enable the experimental
accessibility tree.

Testing: No testing: just adds a pref which is currently unused.
Fixes: part of #4344

Signed-off-by: Alice Boxhall <alice@igalia.com>
Co-authored-by: delan azabani <dazabani@igalia.com>

components/config/prefs.rs

@@ -331,6 +331,8 @@ pub struct Preferences {
     /// Whether or not the viewport meta tag is enabled.
     pub viewport_meta_enabled: bool,
     pub log_filter: String,
+    /// Whether the accessibility code is enabled.
+    pub accessibility_enabled: bool,
 }
 
 impl Preferences {
@@ -416,6 +418,7 @@ impl Preferences {
             dom_worklet_testing_enabled: false,
             dom_worklet_timeout_ms: 10,
             dom_visual_viewport_enabled: false,
+            accessibility_enabled: false,
             fonts_default: String::new(),
             fonts_default_monospace_size: 13,
             fonts_default_size: 16,