Improve generateContentFn helper scriptlet

New directive: `join:[separator][sub-directives]`

[separator] is an author-defined two-character string to be used to split
the following sub-directives string. The sub-directives are fed back into
the helper scriptlet to generate sub-content, which will be joined into
a single string. Example:

...##+js(trusted-prevent-fetch, propstomatch, join:--length:10-20--[some literal content]--length:80-100-- push['ads'])

src/js/resources/utils.js

@@ -220,6 +220,14 @@ export function generateContentFn(trusted, directive) {
             warXHR.send();
         }).catch(( ) => '');
     }
+    if ( directive.startsWith('join:') ) {
+        const parts = directive.slice(7)
+                .split(directive.slice(5, 7))
+                .map(a => generateContentFn(trusted, a));
+        return parts.some(a => a instanceof Promise)
+            ? Promise.all(parts).then(parts => parts.join(''))
+            : parts.join('');
+    }
     if ( trusted ) {
         return directive;
     }