Allow creating directories for downloads path

[cndouglas]

Fixes #10479

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Note: macOS-only

1. Open about:preferences > General tab.
2. Under "Save my downloads here:", click the path field or the pencil icon.
3. Make sure there is a "New Folder" button at the bottom left (see screenshot).

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[NejcZdovc]

++ thank you, works like a charm 😃

[bsclifton]

❤️