Improve validate-constant scriptlet helper

Add support for `json:`-prefixed values.

Author: gorhill

assets/resources/scriptlets.js

@@ -531,7 +531,9 @@ function validateConstantFn(trusted, raw, extraArgs = {}) {
         if ( isNaN(raw) ) { return; }
         if ( Math.abs(raw) > 0x7FFF ) { return; }
     } else if ( trusted ) {
-        if ( raw.startsWith('{') && raw.endsWith('}') ) {
+        if ( raw.startsWith('json:') ) {
+            try { value = safe.JSON_parse(raw.slice(5)); } catch(ex) { return; }
+        } else if ( raw.startsWith('{') && raw.endsWith('}') ) {
             try { value = safe.JSON_parse(raw).value; } catch(ex) { return; }
         }
     } else {