Change Modal.children to be a property, allow instantiating with list of InputText components

[krittick]

Summary

This changes Modal.children to be a property instead of an attribute. This also adds a children parameter to Modal.__init__ to allow instantiating the Modal with list of InputText objects instead of requiring developers to use Modal.add_item

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)

[Dorukyum]

The children parameter could be turned into a variadic argument (*children: InputText) to reflect the items parameter of View

[krittick]

The children parameter could be turned into a variadic argument (*children: InputText) to reflect the items parameter of View

This is a good idea, and I've made the requested change. I believe that makes this PR a breaking change now, but shouldn't matter much since 2.0 is still in beta.