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/test/Problem017.test.js

@@ -1,7 +1,11 @@
 import { countNumberWordLength } from '../Problem017.js'
 
 describe('Number letter count', () => {
-  test.each([[5, 19], [100, 864], [1000, 21124]])('Number letter count from 1 to %i', (n, expected) => {
+  test.each([
+    [5, 19],
+    [100, 864],
+    [1000, 21124]
+  ])('Number letter count from 1 to %i', (n, expected) => {
     expect(countNumberWordLength(n)).toBe(expected)
   })