feat: Test running overhaul, switch to Prettier & reformat everything (#1407)

* chore: Switch to Node 20 + Vitest

* chore: migrate to vitest mock functions

* chore: code style (switch to prettier)

* test: re-enable long-running test

Seems the switch to Node 20 and Vitest has vastly improved the code's and / or the test's runtime!

see #1193

* chore: code style

* chore: fix failing tests

* Updated Documentation in README.md

* Update contribution guidelines to state usage of Prettier

* fix: set prettier printWidth back to 80

* chore: apply updated code style automatically

* fix: set prettier line endings to lf again

* chore: apply updated code style automatically

---------

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Lars Müller <34514239+appgurueu@users.noreply.github.com>

Maths/CountNumbersDivisible.js

@@ -19,13 +19,19 @@
  * @returns {number} count of total number of divisibles
  */
 const countNumbersDivisible = (num1, num2, divider) => {
-  if (typeof num1 !== 'number' || typeof num2 !== 'number' || typeof divider !== 'number') {
+  if (
+    typeof num1 !== 'number' ||
+    typeof num2 !== 'number' ||
+    typeof divider !== 'number'
+  ) {
     throw new Error('Invalid input, please pass only numbers')
   }
 
   // Valid number range is num1 < num2, otherwise throw error
   if (num1 > num2) {
-    throw new Error('Invalid number range, please provide numbers such that num1 < num2')
+    throw new Error(
+      'Invalid number range, please provide numbers such that num1 < num2'
+    )
   }
 
   // if divider is out of range then return 0