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>

Project-Euler/Problem013.js

@@ -1,8 +1,8 @@
 /**
  * Work out the first ten digits of the sum of the following one-hundred 50-digit numbers.
-*/
+ */
 
-export function largeSum (bignum) {
+export function largeSum(bignum) {
   const nums = []
   for (let i = 0; i < bignum.length; i += 50) {
     nums.push(bignum.slice(i, i + 50))
@@ -13,11 +13,11 @@ export function largeSum (bignum) {
   let num = 0
 
   while (pos--) {
-    for (let i = nums.length; i--;) {
+    for (let i = nums.length; i--; ) {
       num += +nums[i].charAt(pos)
     }
-    ret = num % 10 + ret
-    num = num / 10 | 0
+    ret = (num % 10) + ret
+    num = (num / 10) | 0
   }
 
   if (num > 0) {