Use npm's xterm

What does this MR do?

• Remove glTerminal from gl scope
• Remove xterm vendor directory in favor of npm versioned xterm
• Remove vendor directory polyfill for TextEncoder and TextDecoder in favor of npm versioned polyfill

Are there points in the code the reviewer needs to double check?

Shouldn't be

Why was this MR needed?

Reduce vendor files in favor of npm package control

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/39072