Improve ISC of scheduled functions

[danez]

At the moment, we try to analyze a function and figure out if it is scheduled or not. If we cannot find the call to schedule() or cannot find the cron expression, we assume it is not a scheduled function. This sometimes leads to scheduled functions not being detected as such, but instead of erroring we continue and deploy it as a normal function.

To improve this we should make some changes that improve the UX:

1. Simplify detection of scheduled functions. Maybe even just checking if the schedule helper is imported and if so, mark the function as scheduled. The idea is to make this as simple as possible and choose a characteristic of a scheduled function that we can nearly 100% of the time detect.
2. Once the function is marked as scheduled, start searching for the actual cron expression. If we are not able to find it error and let the user know that our code detection wasn't able to find the schedule expression.

[jenae-janzen]

Closed with #1123