Add File menu entry to set Margin as default Markdown editor

[francip]

Problem

Users who want Margin to open .md files by default must go through Finder (Get Info, default-app picker) for every file type. There is no in-app way to make the change.

Design

• Adds File > Set as Default Markdown Editor, wired to a new AppDelegate.setAsDefaultMarkdownEditor(_:).
• The handler calls NSWorkspace.setDefaultApplication(at:toOpen:completion:) (macOS 12+) for the md UTType (UTType(filenameExtension: "md") → net.daringfireball.markdown), then shows a success or failure alert; the failure alert points at the manual Finder path.
• Info.plist already declares the Editor role for md (plus markdown, mdown, mkd, txt) with LSHandlerRank: Alternate, so no plist change is needed; the running app is already registered with Launch Services.
• No work is added to application launch or ordinary file opening; the Launch Services call runs only when the user invokes the menu entry.

Tests

• New WorkspaceBehaviorTests.testFileMenuOffersDefaultMarkdownEditorRegistration asserts the File menu entry exists and is wired to the delegate action.
• Commands run: make test (173 tests, 0 failures) and make smoke (bundle, CLI, document inspection, and app window launch all pass; smoke built with the full Xcode toolchain).

Screenshot

Docs

• Docs/RELEASE_NOTES.md: added an Unreleased entry describing the new menu item.