feat: Add register and verifyEmail methods to useDirectusAuth composable

[M4rk3tt0]

Types of changes

• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR adds modifies the register method and adds the verifyEmail method to the useDirectusAuth composable.

• register: Allows users to register a new account with the Directus register endpoint.
• verifyEmail: Allows users to verify their email address after registration.

These methods were added to address issue #292 and provide a more complete authentication solution. The documentation has also been updated to reflect these changes.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

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