Revamp toolpad file system layout

[Janpot]

Closes #1431
Closes #1817

• It gets the job done by converting the new file format to the old Toolpad app DOM and running Toolpad on that. It's a bit hacky and has a few rough edges that can't be solved in this setup, but the alternative is a big rewrite. We can rewrite piece by piece this way, but already enjoy the new pages folder.
• Use ./pages/<name>/page.yml to store page configuration
• Using zod schemas to validate configuration
• Simplify the datastructure:
  • Make most properties optional, to have it as light as possible
  • Expand the nodes to have a hierarchical structure instead of the flat list of nodes linked by ids
  • Simplify bindings. const values will just be the javascript primitive. Bindings will be an object with { $$jsExpression : '...' }.
• ./.toolpad-generated/ folder moved to ./toolpad/.generated/
• ./.toolpad-generated/.gitignore moved to ./toolpad/.gitignore
• ./toolpad/queries.ts moved to ./toolpad/resources/functions.ts
• added ./toolpad/theme.yml

To Do

• Nest ./queries.ts in these folders
• Query bindings
• ids are constantly being regenerated, resulting in UI losing selection constantly, amongst other weird problems
• Initialization is brittle and has lots of corner cases. Migrations start to become more and more complex and I'm thinking of making this more explicit. e.g.:
  • If toolpad config version < installed toolpad version: Show message "Please run npx toolpad migrate to upgrade the project"
  • If toolpad config version > installed toolpad version: Show message "Please upgrade toolpad"
  • Add explicit CLI command npx toolpad migrate that migrates the project to the currently installed version
• Add version and kind fields