WIP: Add Vue private method naming convention to docs

What does this MR do?

Updates the JS Style Guide docs with the $_ naming convention for "private" Vue methods. (See official Vue style guide)

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Conform by the code review guidelines
  • [ ] Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [ ] Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)