Add selection variables to web implementation

[Skalakid]

Details

This PR:

• adds selection prop support on web
• fixes selectionStart and selectionEnd variables behavior inside MarkdownTextInput ref on web
• fixes selection detection on web

Related Issues

GH_LINK

Manual Tests

1. Start writing in MarkdownTextInput on web
2. Verify if cursor position is correct when writing casually
3. Verify if text replacement works properly. Select some text and start writing
4. Verify if after pasting text or emojis cursor is being set in correct position
5. Add onSelectionChange handler to MarkdownTextInput and verify if event returns correct selection start value
6. Add ref to MarkdownTextInput and verify if ref.selectionStart and ref.selectionEnd returns correct selection end value

Linked PRs

[tomekzaw]

Looks good to me!