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>

Conversions/DateToDay.js

@@ -13,7 +13,15 @@
 */
 
 // Array holding name of the day: Saturday - Sunday - Friday => 0 - 1 - 6
-const daysNameArr = ['Saturday', 'Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']
+const daysNameArr = [
+  'Saturday',
+  'Sunday',
+  'Monday',
+  'Tuesday',
+  'Wednesday',
+  'Thursday',
+  'Friday'
+]
 
 const DateToDay = (date) => {
   // firstly, check that input is a string or not.
@@ -53,7 +61,14 @@ const DateToDay = (date) => {
   Without the adaption, the formula yields `weekDay = -6` for the date 2/3/2014;
   With the adaption, it yields the positive result `weekDay = 7 - 6 = 1` (Sunday), which is what we need to index the array
   */
-  const weekDay = (day + Math.floor((month + 1) * 2.6) + yearDigits + Math.floor(yearDigits / 4) + Math.floor(century / 4) + 5 * century) % 7
+  const weekDay =
+    (day +
+      Math.floor((month + 1) * 2.6) +
+      yearDigits +
+      Math.floor(yearDigits / 4) +
+      Math.floor(century / 4) +
+      5 * century) %
+    7
   return daysNameArr[weekDay] // name of the weekday
 }