Resolve "Support creating new file from URL in the Web IDE"

What does this MR do?

Allows creation of the files by simply supplying the file name in URL like https://gitlab.com/-/ide/project/<group>/<project>/edit/<branch>/-/<new_file_name>.

This resolves https://gitlab.com/gitlab-org/gitlab-ce/issues/57668.

Note! This MR supports only creation of the new files in existing branches. Though, when combined with https://gitlab.com/gitlab-org/gitlab-ce/issues/45687 it should be possible to create both, branch and a new file with just a URL!

This MR is the logical step before solving https://gitlab.com/gitlab-org/gitlab-ce/issues/59461

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/57668 https://gitlab.com/gitlab-org/gitlab-ce/issues/45687

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #57668 (closed)