[mv3] Support conditional `env_mv3` directive to parser

Related issue:
- https://github.com/uBlockOrigin/uBOL-issues/issues/1

platform/mv3/make-rulesets.js

@@ -57,6 +57,7 @@ const rulesetDir = `${outputDir}/rulesets`;
 const scriptletDir = `${rulesetDir}/js`;
 const env = [
     'chromium',
+    'mv3',
     'native_css_has',
     'ublock',
     'ubol',

src/js/static-filtering-parser.js

@@ -3147,6 +3147,7 @@ Parser.utils = Parser.prototype.utils = (( ) => {
         [ 'env_firefox', 'firefox' ],
         [ 'env_legacy', 'legacy' ],
         [ 'env_mobile', 'mobile' ],
+        [ 'env_mv3', 'mv3' ],
         [ 'env_safari', 'safari' ],
         [ 'cap_html_filtering', 'html_filtering' ],
         [ 'cap_user_stylesheet', 'user_stylesheet' ],