import-content-task-globals

For files containing ContentTask.spawn calls, this will automatically declare the frame script variables in the global scope. ContentTask is only available to test files, so by default the configs only specify it for the mochitest based configurations.

This saves setting the file as a mozilla/frame-script environment.

Note: due to the way ESLint works, it appears it is only easy to declare these variables on a file global scope, rather than function global. This may mean that they are incorrectly allowed, but given they are test files, this should be detected during testing.