Adding Async/Defer to the JSConcat script

There does not seem to be a way to filter the resulting script block. If it could be filtered then attributes could be added to the concatenated script block such as performance related `async` or `defer`.

The issue does not name a file, test, or entry point. Start by locating the JSConcat script-block generation in the plugin and inspect existing filtering or hook patterns. Define how callers would supply attributes such as async or defer, then add coverage showing that the concatenated script block receives them.

Written by the indexing model from the issue text.