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

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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/Problem002.test.js

@@ -2,7 +2,9 @@ import { EvenFibonacci } from '../Problem002'
 
 describe('Even Fibonacci numbers', () => {
   it('should throw error when limit is less than 1', () => {
-    expect(() => EvenFibonacci(-1)).toThrowError('Fibonacci sequence limit can\'t be less than 1')
+    expect(() => EvenFibonacci(-1)).toThrowError(
+      "Fibonacci sequence limit can't be less than 1"
+    )
   })
   test('when limit is greater than 0', () => {
     expect(EvenFibonacci(40)).toBe(44)