Create shared configuration builders.

[Hug0-Drelon]

What?

Let's refactor webpack.config.js from all our plugins.

How?

Create new API to generate webpack configurations:

• getVanillaConfig for Vanilla JS and CSS files (meaning no complex build rules nor JSX transpilation).
• getReactifiedConfig for React-like JS and CSS files (perfect for custom sidebars or blocks for instance).
• Cover it with Jest unit tests.
• Take the opportunity to add ESLint and a proper CI.
• Lint already existing files and rename them (their inner code is kept untouched).

[Hug0-Drelon]

@Marianne380 I've added SASS transpilation in reactified.js (see sassRulesMinified and sassRulesUnminified). Sorry for the modification during your review... 😅

[Marianne380]

Otherwise, LGTM.