Fix types for services

[pako88]

Proposed changes

Fixed Types for services.

Without this fix:
If you import a simple custom service class (without an own package) like described here https://webdriver.io/docs/customservices.html on a TypeScript project it will fail due that the service isn't a string.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• I have added proper type definitions for new commands (if appropriate)

Further comments

Reviewers: @webdriverio/project-committers

[christian-bromann]

👍 good catch!