feat: create test playground + introduce navigation

[eszlamczyk]

What/Why?

First phase of setting up Maestro E2E tests for
react-native-enriched-markdown. To make automated testing
feasible, a dedicated PlaygroundScreen was added to the
example app as a stable, testable surface. Additionaly introduced react-navigation to the app to make example app better to read.

React Navigation

To incorporate react navigation introduced Home screen that includes routing buttons for each available example screen (Playground, Text, Input, Stream). On top of that added simple header informing about current screen and giving ability to go back to home screen.

Playground

The playground includes:
  - Focus / Blur / Clear controls for the editor
  - Size toggle (base <-> max height) to test dynamic layout
  - Formatting toolbar with Bold, Italic, Underline,
  Strikethrough, Spoiler, and Link toggles
  - Link modal for inserting/editing links
  - "Get Raw Markdown" button that shows the current markdown
  via an alert
  - Live preview panel using EnrichedMarkdownText that
  reflects editor content (he made by typing using either features of EnrichedMarkdownTextInput or *normal markdown syntax*) in real time

Testing

Run the example app on Android or iOS and navigate to the
Playground screen. Verify that:
  - The formatting toolbar correctly applies and removes
  styles
  - The link modal opens, accepts input, and inserts/removes
  links
  - The size toggle resizes the editor
  - "Get Raw Markdown" displays the current content
  - The preview panel updates live as you type

Screenshots

	Android	IOS
Home screen
New Header (example in text view)
Playground

Input element screenshot

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

[hryhoriiK97]

Overall looks really nice - we are moving in the right direction. I've left a few comments, please take a look 🙏