feat(includes): add only field to limit which tasks are included

[JakesMD]

What changed

• Added only field to includes as the inverse of excludes:

  includes:
    shared:
      taskfile: shared.yml
      only: [foo, baa]

• Added tests
• Updated guide.md, schema.md and schema.json

Use cases

• include specific task from a "third party" or even a remote taskfile
• avoid having to keep updating excludes whenever tasks are added to included taskfile
• most useful when internal: false and the included tasks are part of the "public API"

Disclosure

• AI was used during implementation
• I have read and followed the Contribution Guide

[JakesMD]

Hi! I was wondering if there's a specific reason this hasn't been implemented before. When you only want to include a small subset of tasks from a larger taskfile, an only option makes more sense than a long excludes list.

You could split the included taskfile into multiple smaller ones, but for remote taskfiles that might not be an option. And when multiple taskfiles depend on an overlapping subset of tasks, that doesn't work either. The included taskfile also shouldn't need to know how it's being used, so even filtering by prefix or glob pattern isn't a good fit.

I'm not sure about the field name only. Other options could be includes, select(s) or subset.