📝 Docs: Standardize slogan as something without "simple"

[JoshuaKGoldberg]

Documentation Request Checklist

• I have read and agree to Mocha's Code of Conduct and Contributing Guidelines
• I have searched for related issues and issues with the faq label, but none matched my issue.
• I want to provide a PR to resolve this

Overview

Words like "easy" and "simple" are often used in writing to convey that something doesn't require extra complications or tooling. But they also mean in English that something should be literally "easy" or "simple" - which often isn't the case for devs new to a technology. It's good writing practice to use more precise adjectives instead.

Right now the Mocha slogan is "Simple, flexible, fun JavaScript test framework for Node.js & The Browser":

mocha/README.md

Line 5 in 645469e

<p align="center">☕️ Simple, flexible, fun JavaScript test framework for Node.js & The Browser ☕️</p>

Let's use a slogan that doesn't say "simple". Maybe just "flexible and fun" is enough?

Additional Info

https://justsimply.dev is a good resource that goes into more detail and has examples.

[JoshuaKGoldberg]

Food for thought, folks occasionally express a need for cross-JS-runtime tests. Example from https://twitter.com/_nicojs/status/1762508703863894472:

Nicholas C. Zakas @slicknet
What I want: To write JavaScript tests once and be able to run them across Node.js, Bun, and Deno.
Problem: Bun and Deno have built-in test runners you have to import from to run tests. I use Mocha. This doesn't work.
Solution: ???