Add top-lvl support in Wasp for forms.

[Martinsos]

Defining forms is often painful and boilerplatish, and on the other hands it is mostly declarative, so it makes a lot of sense to have direct support for forms in Wasp, in declarative fashion.

We were already experimenting somewhat with it before, so it might be worth taking a look at that code for inspiration.

There are two levels to this:

1. Defining any form.
2. Defining forms for editing entity.

Both are good candidates as Wasp features.
This certainly needs more investigation.

[Martinsos]

Potentially interesting: https://github.com/iway1/react-ts-form -> library for reducing boilerplate when doing forms in React.