cleanup and consistency (#213)

- Made the use of `const` and `let` consistent
- import required functions and types from 'react' instead of using the
  `React.` namespace.
- Added `Expand` type, which can expand complex types to their "final"
  result.
- Ensured that we use `as const` for DEFAULT_XXX_TAG where we used a
  string. So that we have the type of `div` instead of `string` for
  example.
- Used `interface` over `type` where possible. I'm personally more of a
  `type` fan. But the TypeScript recommends `interfaces` where possible
  because they are faster, yield better error messages and so on.

jest/custom-matchers.ts

@@ -1,17 +1,17 @@
 import '@testing-library/jest-dom/extend-expect'
 
 // Assuming requestAnimationFrame is roughly 60 frames per second
-const frame = 1000 / 60
-const amountOfFrames = 2
+let frame = 1000 / 60
+let amountOfFrames = 2
 
-const formatter = new Intl.NumberFormat('en')
+let formatter = new Intl.NumberFormat('en')
 
 expect.extend({
   toBeWithinRenderFrame(actual, expected) {
-    const min = expected - frame * amountOfFrames
-    const max = expected + frame * amountOfFrames
+    let min = expected - frame * amountOfFrames
+    let max = expected + frame * amountOfFrames
 
-    const pass = actual >= min && actual <= max
+    let pass = actual >= min && actual <= max
 
     return {
       message: pass