Avoid opening the source view when double-clicking JS functions, while JS is still unsupported

At the moment, the source view doesn't support JavaScript code at all:

1. The line timings are wrong until [bug 1441689](https://bugzilla.mozilla.org/show_bug.cgi?id=1441689) is fixed.
1. We don't have a way of fetching the JS code.

As long as this is the case, it's undesirable to open the source view from JS functions. Maybe we should just ignore double clicks on them for now

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-489)