E2E test: wait for file to load in IDE

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/467729

Waits for the README.md file to be loaded within the web ide file explorer before proceeding with the test. In https://gitlab.com/gitlab-org/gitlab/-/issues/467729#note_1957650816 we observed that the right click menu doesn't have all the options before when vscode is loading and files aren't yet loaded in the file explorer.

I think it's reasonable to wait for the project sidebar to be loaded before proceeding with this test.

Also some refactoring. (moved some methods to private, added waiting messages)

How to set up and validate locally

bundle exec rspec qa/specs/features/browser_ui/3_create/web_ide/upload_new_file_in_web_ide_spec.rb