fix: deepMerge util

[jakex7]

Fixes the types and functionality of deepMerge util.

Changes:

• Fix types reported in fix: mute ts error for deepMerge #165
• Introduce simple tests to deepMerge
  • merge theme
  • merge styles

---

In the previous implementation, deepMerge styles didn't work with arrays and tried to merge even nested styles. Currently, it only merges to the level that is in the source. Example:

{
  container: { backgroundColor: '#000' },
  emoji: {
    selected: {
      backgroundColor: '#000',
      transform: [{ rotate: '45deg' }],
    },
  },
}

[DominikDanielewicz]

All good!