feat: add getCaretRect command and onCaretRectChange event

[hryhoriiK97]

What/Why?

Fixes: #233

Add onCaretRectChange callback and imperative getCaretRect() command to EnrichedMarkdownInput.

When the input is used with scrollEnabled={false} inside an outer ScrollView (e.g., a post composer), there's no built-in way to know where the caret is in pixel coordinates — onChangeSelection only provides character offsets, which aren't enough to determine screen position with word-wrapped lines.

onCaretRectChange fires whenever the caret moves (typing, selection change, content reflow) and returns { x, y, width, height } in dp relative to the input's top-left corner. The native side diffs the rect before emitting to avoid redundant bridge traffic. getCaretRect() provides the same data as a one-off Promise-based query.

Both APIs surface caret geometry that the native text systems already track internally (UITextView.caretRectForPosition on iOS, Layout.getLineForOffset/getLineTop/getPrimaryHorizontal on Android).

Testing

PR Checklist

• Code compiles and runs on iOS
• Code compiles and runs on Android
• Updated documentation/README if applicable
• Ran example app to verify changes