feat: add global precondition

[vmaerten]

Fixes #294

It's a fairly popular feature

[vmaerten]

I was wondering how it should behave if an included Taskfile defines global preconditions:

1. Should it produce an error (similar to dotenv)?
2. Should all preconditions be merged and applied together?
3. Should the global preconditions in the included Taskfile be ignored?

I started with the 2 in mind, but I am not sure anymore

Any thought ? cc @pd93 / @andreynering

[pd93]

As a user, I would expect 2. This is also the safer option if we maintain the principle of "its safer to not run a task that was intended to run than it is to run one that wasn't."

[vmaerten]

I followed the same pattern we have with vars.
I am not a huge fan of naming preconditions inside preconditions, maybe values ?
I you have some naming ideas