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>

Conversions/UpperCaseConversion.js

@@ -17,7 +17,7 @@
  */
 const upperCaseConversion = (inputString) => {
   // Take a string and split it into characters.
-  const newString = inputString.split('').map(char => {
+  const newString = inputString.split('').map((char) => {
     // Get a character code by the use charCodeAt method.
     const presentCharCode = char.charCodeAt()
     // If the character code lies between 97 to 122, it means they are in the lowercase so convert it.