Allow integrating toolpad applications in a custom server

[Janpot]

Export a unstable_createHandler which can be used to integrated in a custom server. See example.

Can be imported as ESM, should also work under commonjs. In any case it can always dynamically be imported as ESM under commonjs.

Changes include:

• make serverside code ESM compatible
• refactor the intiialization of options so that it can be shared between cli and custom server
• export unstable_createHandler
• remove projectRoot. (dead code)
• disable the preview header in custom server (for now at least). It's not ideal because:
  • It contains the theme switcher for the editor. Which theme should it choose? Does it even make sense in the first place to have the editor theme in the preview window?
  • contains a button to open the editor, while there is nothing to open (yet)

Follow up work includes:

• Allow editing apps running under custom server

[Janpot]

Putting up for rereview. I turned this PR into exporting the toolpad handler as it turned out there were more things that needed to be done than was initially proposed

[apedroferreira]

So in the example /my-app is the path that the whole Toolpad runs under including the editor in dev mode, and /my-app is the base path where the production app runs?
If so, should they be different to be more obvious?

[Janpot]

including the editor in dev mode

Not the editor, only the application. For editing I'm envisioning the following:

• you start your custom server with the Toolpad handler in dev mode.
• you run yarn toolpad --connect http://localhost:3000/my-app
• then follow the link that's printed to open the editor where the canvas is actually pointing to your custom server

Only the end user application runs in your server

[apedroferreira]

I see, that sounds cool!
Are there cases where the first argument for app.use and appBase would not be the same, though? Or could they share the same variable?
Anyway that's not important at all so I'm approving already.

[Janpot]

it's an example. for demonstration purposes is it a bit clearer.

[apedroferreira]

Just noticed a couple of things maybe worth mentioning, everything else looks good!

[Janpot]

I'm getting an error for a missing exports property in package.json when trying to run the custom server example.

How are you running it? The following should work:

• add examples/custom-server to the package.json workspaces field
• run yarn && yarn dev --ignore custom-server
• run yarn workspace custom-server dev