feat: add thumbSize to set size of thumb

[bdtren]

Summary:

This PR adds a new thumbSize prop to control the thumb’s size for both with and without thumb image. It is a simple feature that I need and did patch-package in some of my projects but I didn't have a chance to contribute, I think some developers might also need it:

Android	iOS

Test Plan:

• Install/refresh package:

npm install
npm run refresh-package

• Android:

npm run example-android

Verify in the example list:
“Custom thumb size (no image)” renders a larger thumb (e.g. thumbSize={32}) tinted with thumbTintColor.
“Custom thumb size (scaled image)” renders the thumb image scaled to the configured size (e.g. thumbSize={60}).

• iOS:

npm run example-ios

Verify the same two examples behave equivalently on iOS (thumb resizes; image scales when provided).