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>

String/test/FormatPhoneNumber.test.js

@@ -3,7 +3,9 @@ import formatPhoneNumber from '../FormatPhoneNumber'
 describe('Testing the formatPhoneNumber functions', () => {
   it('expects to throw a type error', () => {
     expect(() => formatPhoneNumber('1234567')).toThrow('Invalid phone number!')
-    expect(() => formatPhoneNumber('123456text')).toThrow('Invalid phone number!')
+    expect(() => formatPhoneNumber('123456text')).toThrow(
+      'Invalid phone number!'
+    )
     expect(() => formatPhoneNumber(12345)).toThrow('Invalid phone number!')
   })