Add command to create a Swift DocC documentation catalog

[amanthatdoescares]

Description

Add command to create DocC documentation catalog (#1647)

• Add Swift: Create DocC Documentation Catalog command that lets users
  select one or more SwiftPM targets and creates a .docc catalog inside
  each target's directory (SourceKit-LSP discovers docs there).
• Module markdown uses title format # ModuleName per maintainer.
• Register command in register(WorkspaceContext); add unit tests.

This removes the need to manually set up the DocC folder structure and makes it easier to start documenting Swift packages directly from VS Code.

Issue: #1647

Tasks

• Required tests have been written
• Documentation has been updated (not applicable)
• Added an entry to CHANGELOG.md if applicable

[matthewbastien]

Thank you for this PR! There are a few issues that need to be resolved before this can be merged.

[matthewbastien]

I'd also like to have an integration test for this since the logic will be interacting with the FolderContext. Something that runs the command and makes sure that all of the targets show up correctly in the quick pick. It should also make sure that the catalog gets created properly.

[amanthatdoescares]

@matthewbastien I’ve added an integration test that runs the command end-to-end, verifies SwiftPM targets appear in the QuickPick, and asserts the DocC catalog is created on disk.

This is my first time writing an integration test in this codebase, so I’d really appreciate a review on the test structure and coverage.

[matthewbastien]

Looking much better, but there are a bunch of areas where the code is more complicated than it needs to be.

[amanthatdoescares]

@matthewbastien i have implemented the changes and it is ready for another review.