Does not support validating Enterprise SSO users

[lukasz-mitka]

Description
Validation fails on checking valid enterprise sso users
Example: @user-name-can-be-very-long_and-has-underscore

Validation fails after adding SSO user to CODEOWNERS file.

Expected result
User should be validated successfully.

Actual result
Fails with

Owner @user_id does not look like a GitHub username or team name

Steps to reproduce
Add * @user_id to CODEOWNERS file

Troubleshooting
Here's the regex that doesn't match it.

codeowners-validator/internal/check/valid_syntax.go

Line 16 in 84b4fa2

usernameOrTeamRegexp = regexp.MustCompile(`^@(?i:[a-z\d](?:[a-z\d-]){0,37}[a-z\d](/[a-z\d](?:[a-z\d_-]*)[a-z\d])?)$`)

[lukasz-mitka]

I just noticed I only fixed the _ part of this issue in #209.
The unlimited username length will still be an issue :(