feat: Add required selections and exclusive options to forms

[FXschwartz]

Adds two form rules needed for serverpod/serverpod#5788, where serverpod create shows the IDE/agent screen first and must not let the user continue until they have selected something. The rules live in the form itself, so every way of advancing (Enter, Space on a focused Next, mouse click) is covered.

• FormSelectionConfig.selectionRequired: a multi-screen form will not move past the config until at least one option is selected.
• FormSelectionConfig.exclusiveOptions: multi-select options that can not be combined with any other option, such as "None".
• MultiScreenFormState.canAdvance exposes whether the current screen can be left, and the Next/Submit button is disabled while it can not.
• Bumps the version to 0.13.0.

Breaking: implementers of FormSelectionConfig must add exclusiveOptions and selectionRequired.

[marcelomendoncasoares]

LGTM!