Exclude external userspace from lint checking

[zvecr]

Description

Updated qmk lint default behaviour of checking "default-ish" keymaps to ignore userspace.

There are a few things to consider,

• someone might want to actually lint userspace keymaps?
  • not a ton of value right now, but with the recent keymap.json changes will become more important
  • can still do qmk lint -kb whatever -km my_important_keymap
• -km all behaviour?
  • currently still checks userspace
  • would prefer not to have to add an additional --userspace/--no-userspace arg

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Issues Fixed or Closed by This PR

• Fixes [Feature Request] Exclude external userspace from lint checking #24676

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).