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>

Data-Structures/Stack/StackES6.js

@@ -10,19 +10,19 @@
 
 // Class declaration
 class Stack {
-  constructor () {
+  constructor() {
     this.stack = []
     this.top = 0
   }
 
   // Adds a value to the end of the Stack
-  push (newValue) {
+  push(newValue) {
     this.stack.push(newValue)
     this.top += 1
   }
 
   // Returns and removes the last element of the Stack
-  pop () {
+  pop() {
     if (this.top !== 0) {
       this.top -= 1
       return this.stack.pop()
@@ -31,17 +31,17 @@ class Stack {
   }
 
   // Returns the number of elements in the Stack
-  get length () {
+  get length() {
     return this.top
   }
 
   // Returns true if stack is empty, false otherwise
-  get isEmpty () {
+  get isEmpty() {
     return this.top === 0
   }
 
   // Returns the last element without removing it
-  get last () {
+  get last() {
     if (this.top !== 0) {
       return this.stack[this.stack.length - 1]
     }
@@ -49,7 +49,7 @@ class Stack {
   }
 
   // Checks if an object is the instance os the Stack class
-  static isStack (el) {
+  static isStack(el) {
     return el instanceof Stack
   }
 }