[Single file editor] Include label element for file name input, even if visually hidden

Problem to solve

This issue represents a finding from our work to audit features belonging to groupsource code. See the analysis in Figma for more information. See the overview video for guidance on how to review this analysis.

Screenshots

Input	WAVE error	Accessibility tree

Proposal

• Address the following finding on the Single file editor page:
  • Include label element for file name input, even if visually hidden (https://www.w3.org/WAI/WCAG22/Understanding/labels-or-instructions.html)
• Additional notes and guidance to resolve this finding:
  • Missing a label as part of GlFormGroup. If a visible label isn't desired, then it should only be visibly hidden, but still available in code.
• Finding yellow #3