Use `.native.tsx` instead of `.web.tsx`

React web apps don't use the `.web.tsx` pattern. However, all native apps do use platform extensions.

I'd propose changing `MarkdownTextInput.tsx` to have the web code, and `MarkdownTextInput.tsx` to have the React Native code. This way, Web developers don't need additional config to install the library.

I'm happy to do this in a PR if you'd merge it.