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/Problem025.test.js

@@ -1,27 +1,27 @@
-import { fibonacciIndex } from '../Problem025'
-
-describe('Check Problem 25 - 1000 digit Fibonnaci number', () => {
-  it('First term of the Fibonnaci sequence containing 3 digits', () => {
-    expect(fibonacciIndex(3)).toBe(12)
-  })
-
-  it('First term of the Fibonnaci sequence containing 10 digits', () => {
-    expect(fibonacciIndex(10)).toBe(45)
-  })
-
-  it('First term of the Fibonnaci sequence containing 50 digits', () => {
-    expect(fibonacciIndex(50)).toBe(237)
-  })
-
-  it('First term of the Fibonnaci sequence containing 100 digits', () => {
-    expect(fibonacciIndex(100)).toBe(476)
-  })
-
-  it('First term of the Fibonnaci sequence containing 1000 digits', () => {
-    expect(fibonacciIndex(1000)).toBe(4782)
-  })
-
-  it('First term of the Fibonnaci sequence containing 10000 digits', () => {
-    expect(fibonacciIndex(10000)).toBe(47847)
-  })
-})
+import { fibonacciIndex } from '../Problem025'
+
+describe('Check Problem 25 - 1000 digit Fibonnaci number', () => {
+  it('First term of the Fibonnaci sequence containing 3 digits', () => {
+    expect(fibonacciIndex(3)).toBe(12)
+  })
+
+  it('First term of the Fibonnaci sequence containing 10 digits', () => {
+    expect(fibonacciIndex(10)).toBe(45)
+  })
+
+  it('First term of the Fibonnaci sequence containing 50 digits', () => {
+    expect(fibonacciIndex(50)).toBe(237)
+  })
+
+  it('First term of the Fibonnaci sequence containing 100 digits', () => {
+    expect(fibonacciIndex(100)).toBe(476)
+  })
+
+  it('First term of the Fibonnaci sequence containing 1000 digits', () => {
+    expect(fibonacciIndex(1000)).toBe(4782)
+  })
+
+  it('First term of the Fibonnaci sequence containing 10000 digits', () => {
+    expect(fibonacciIndex(10000)).toBe(47847)
+  })
+})