feat: Add ToXDocument method to TwiML

[Swimburger]

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

Fixes #648

This PR adds the ToXDocument method to the TwiML class.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the Contribution Guidelines and my PR follows them
• I have titled the PR appropriately
• I have updated my branch with the main branch
• I have added tests that prove my fix is effective or that my feature works
• I have added the necessary documentation about the functionality in the appropriate .md file
• I have added inline documentation to the code I modified

[mattcole19]

Thanks for adding this!