qmk new-keymap: validate keymap name

[fauxpark]

Description

Avoids slashes in the keymap name (eg. partial paths).

Types of Changes

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

Issues Fixed or Closed by This PR

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).

[fauxpark]

Build failures are unrelated.

[lesshonor]

While I get the reasoning, forbidding uppercase letters and hyphens strikes me as something end users will repeatedly hit face-first and raise questions/complaints about unless that restriction is clearly noted. (The advice to use your GitHub username is fairly common; though case-insensitive, GH usernames allow uppercase letters while disallowing underscores¹ and allowing hyphens.)

Footnotes

1. Outside of enterprise SSO. ↩

[fauxpark]

I'm not married to the regex, I just took the one used in qmk new-keyboard and removed the slash.
That said, there may be conflicting constraints here between what is allowed as a GitHub username and what we prefer as keyboard/keymap names due to the case sensitivity issue.

Given we're no longer accepting user keymaps into the repo perhaps this is less of an "our problem", but I can see, and have seen, people getting confused that their keymap is not being found because they typed it in with the wrong case.