Allow function tasks to return parallel tasks

Is your feature request related to a problem? Please describe.

While lint-staged now supports parallel tasks by using nested arrays to group tasks that should be run in parallel, function tasks are not allowed to return parallel tasks. They are current limited to returning only sequential tasks, both at a type level and runtime level.

Describe the solution you'd like

Allow function tasks to return parallel tasks.

Describe alternatives you've considered

Continuing to use fake globs as described in https://github.com/lint-staged/lint-staged/issues/934#issuecomment-1097793208

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