Add Support for Dynamic Root-level Form Properties in Vue and Svelte

[flexponsive]

Adds support for dynamic root-level form properties in Vue and Svelte, matching existing React behavior. Properties can be added after initialization, which helps avoid pre-defining every possible field on create pages and supports temporary values like “type your username to confirm.”

Changes

• Update form.data() in Vue and Svelte to track all form properties, including those added at runtime, not only those defined at initialization.
• Update example pages to demonstrate dynamic properties.
• Add Playwright tests for dynamic root-level and nested properties in all adapters.

Motivation
Fixes #968 and follows discussion #859. Ensures consistent behavior across adapters.

[flexponsive]

@pascalbaljet Thanks for all the recent work on improving the forms, especially the TypeScript support — the new type checks for form fields are a big robustness win.

That said, in JavaScript apps the long-standing issue remains: form fields added at runtime are still silently dropped in data() for Vue and Svelte. This is orthogonal to the TypeScript improvements and could be resolved with a very small code change, as suggested here.

Would you be open to a PR along these lines? I can take care of the test app updates across all adapters and ensure no backward-compatibility issues.