Ability to validate a taskfile

[pioio]

It would be useful to validate (e.g. after making changes, pre-commit) the syntax of a taskfile (e.g. task --validate Taskfile.yaml) without needing to execute all commands in it in a dry-run mode.

By default, it the yaml has unknown keys, this should be a validation error. That's because it's a likely typo.
However, and this is just a nice to have, we should consider allowing to ignore such error (by turning them into warning) in case someone is using non-standard keys intentionally, e.g. for external tools also reading from Taskfile.yaml. But again, this part is merely something to consider.