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/Problem001.test.js

@@ -2,10 +2,14 @@ import { multiplesThreeAndFive } from '../Problem001.js'
 
 describe('Sum of multiples of 3 or 5', () => {
   it('should throw error when number is negative number', () => {
-    expect(() => multiplesThreeAndFive(-24)).toThrowError('No natural numbers exist below 1')
+    expect(() => multiplesThreeAndFive(-24)).toThrowError(
+      'No natural numbers exist below 1'
+    )
   })
   it('should throw error when number is 0', () => {
-    expect(() => multiplesThreeAndFive(0)).toThrowError('No natural numbers exist below 1')
+    expect(() => multiplesThreeAndFive(0)).toThrowError(
+      'No natural numbers exist below 1'
+    )
   })
   test('if the number is greater than 0', () => {
     expect(multiplesThreeAndFive(10)).toBe(23)