PlainForm and IconForm Edit, DropdownForm Initialization, Spacer Props Addition and Edit, IconForm and Spacer Tests, Package Addition

1. Edit the icon position in IconForm to center, fix text height
2. Fix the text height in PlainForm
3. DropdownForm Initialization
4. Added tests for IconForm and Spacer component
5. Added tests library from react-native and jest to Package.json
6. Edit Spacer props and replace them with several new props:

• "s" for height = 4
• "m" for height = 8
• "l" for height = 16
• "xl" for height = 24