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/ZellersCongruenceAlgorithm.js

@@ -1,6 +1,10 @@
 // Zeller's Congruence Algorithm finds the day of the week from the Gregorian Date. Wikipedia: https://en.wikipedia.org/wiki/Zeller%27s_congruence
 export const zellersCongruenceAlgorithm = (day, month, year) => {
-  if (typeof day !== 'number' || typeof month !== 'number' || typeof year !== 'number') {
+  if (
+    typeof day !== 'number' ||
+    typeof month !== 'number' ||
+    typeof year !== 'number'
+  ) {
     throw new TypeError('Arguments are not all numbers.')
   }
   const q = day
@@ -11,8 +15,13 @@ export const zellersCongruenceAlgorithm = (day, month, year) => {
     y -= 1
   }
   day =
-      (q + Math.floor(26 * (m + 1) / 10) + (y % 100) + Math.floor((y % 100) / 4) + Math.floor(Math.floor(y / 100) / 4) + (5 * Math.floor(y / 100))) %
-          7
+    (q +
+      Math.floor((26 * (m + 1)) / 10) +
+      (y % 100) +
+      Math.floor((y % 100) / 4) +
+      Math.floor(Math.floor(y / 100) / 4) +
+      5 * Math.floor(y / 100)) %
+    7
   const days = [
     'Saturday',
     'Sunday',