test(editor): add fixture based markdown round trip tests

[JammingBen]

Description

Stacked on #3614.

Moves the markdown round trip tests of the text editor to fixture files in packages/web-pkg/tests/unit/editor/strategies/fixtures/markdown/. This makes them more transparent while also providing files a dev can work with. Adding a case now means adding a .md file.

Each fixture is checked for:

• exact output: the saved markdown equals the fixture, or <name>.out.md
• stability: saving the saved markdown again changes nothing
• rendering: marked renders the original and the saved markdown the same

Fixtures in unlike-marked/ skip the render check, since the editor reads them differently from marked on purpose (empty item below text, task syntax in ordered lists, under or tab indented content below task items, indented tilde fence, list starting at 0).

Also:

• registers the markdown tokenizers on a Marked instance type instead of typeof marked
• moves the shared test helpers to helpers.ts
• keeps only the inline tests that can't be fixtures (editor document input, parsed document checks, performance, Marked instance)

Issue

fixes #3620