Add `submitBehavior` prop support for MarkdownTextInput on web

[daledah]

Summary

React Native has deprecated the blurOnSubmit prop for TextInput. Migrate deprecated blurOnSubmit to submitBehavior.

Problem

The submitBehavior prop is a modern replacement for the deprecated blurOnSubmit prop in React Native's TextInput component. However, MarkdownTextInput on web currently only supports blurOnSubmit, which prevents applications from using the newer API.

The submitBehavior prop provides more granular control over submission and blur behavior:

• 'submit': Submit the form without blurring
• 'blurAndSubmit': Submit the form and blur the input
• 'newline': Insert a newline instead of submitting (default behavior for multiline inputs)