Bug 1720570 - Add pref for css module scripts; r=layout-reviewers,emilio

Differential Revision: https://phabricator.services.mozilla.com/D263408

Author: dminor

modules/libpref/init/StaticPrefList.yaml

@@ -9720,6 +9720,13 @@
   mirror: always
   rust: true
 
+# Whether to enable CSS Module Scripts support.
+# https://html.spec.whatwg.org/#creating-a-css-module-script
+- name: layout.css.module-scripts.enabled
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
 # Override DPI. A value of -1 means use the maximum of 96 and the system DPI.
 # A value of 0 means use the system DPI. A positive value is used as the DPI.
 # This sets the physical size of a device pixel and thus controls the