deno.json workspace support for Deno 2

[dsherret]

Tracking issue for unstable workspaces:

• Workspace should be detected when cwd is in a package of a workspace #22020
• LSP can't resolve import_maps in workspaces when using --unstable-workspaces #21769
•  node: Unable to resolve modules from workspace #18546
• Make properties in deno.json work well based on top level workspace
• Make it work well with npm packages
• Update documentation suggestion: document workspaces functionality deno-docs#415

[marvinhagemeister]

Adding for completeness: I think the setup in Fresh is a bit odd given that the website is located inside the folder structure of Fresh itself.

<repo>/  # <- Fresh source here
  www/ # <- inside fresh/ folder

I would assume that we would switch to a each having their own directory when moving to workspaces:

<repo>/
  /fresh
  /www

The more important thing for me is if shared dependencies are properly hoisted to the top level workspaces folder to make deduplication work properly.

[bartlomieju]

When implementing this we should move as much code as possible to deno_config so other tools like dnt can reuse it.