[feature] add conditional tasks

[piotrkowalczuk]

Is your feature request related to a problem? Please describe.
I want to be able to skip a task if the codebase does not meet the given criteria at the execution time. For example, in Go, one might want to create a task that executes go generate. The output files are known and should be defined. What's unknown is whether the given project has any //go:generate annotation in its codebase.

tasks:
  generate:
    command: 'go generate ./...'
    inputs:
      - '**/*.go'
      - 'go.mod'
      - '/go.mod'
    outputs:
      - '**/*_templ.go'

This task will produce error if run against Go project without any //go:generate templ generate ./... annotation:

Error: task_runner::missing_outputs

  × Task foo-bar:generate defines outputs but after being ran, either none or not all of them exist.
  │ If you require optional outputs, try using glob patterns instead.

Describe the solution you'd like

Having additional property condition in the Task schema. Value of this field could a simple bash script.

Bash

tasks:
  generate:
    condition: |
      if output=$(go generate -n ./... 2>&1 1>/dev/null); then
        if [[ -n "$output" ]]; then
            exit 1
        else
            exit 0
        fi
      fi

    command: 'go generate ./...'
    inputs:
      - '**/*.go'
      - 'go.mod'
      - '/go.mod'
    outputs:
      - '**/*.pb.go'
      - '**/*_templ.go'

Describe alternatives you've considered

Additional context

[chadrik]

Note that go-task has this feature and it's pretty handy: https://taskfile.dev/usage/#using-programmatic-checks-to-indicate-a-task-is-up-to-date

I needed this feature because the output of my task is outside of the repo, so I can't use outputs. When I was using task I wrote a simple existence check for the output directory and the task would be skipped if it existed. With moon, I have to write a wrapper script around the command that I want to execute to do the check and bail early if the output directory exists.