Convert `jest-preview` to monorepo using `pnpm workspace` and `Nx`

[nvh95]

Is your feature request related to a problem? Please describe.

• Sometimes forgot to build transforms before running test/ demo
• Avoid the leak implementation of PostCSS via postcss.config.js (parent traversal, see website/postcss.config.js and examples/svelte/postcss.config.cjs for more)
• Better structure code base
• Leverage local caching feature of nx
• Leverage cloud caching feature of nx

Describe the solution you'd like

• [ ] Use pnpm instead of npm
• [ ] Use nx for speedup
  • [ ] Ask Nx's folks if they can help reviewing the setup
• [ ] Extract current codebase to multiple packages

Describe how should jest-preview implements this feature

Separate to different packages

• src/ jest-preview
• cli
• demo
• examples (to link to jest-preview in all examples)
• tests (new ones, to contain e2e tests, for e.g: sass < 1.45.0)
• docs-e2e-testing
• website

Describe alternatives you've considered

• turborepo

Additional context

• Lerna is dead — Long Live Lerna
• Reference: https://github.com/strapi/strapi/pull/13664

[nvh95]

TODO for @nvh95 Add other packages:

• jest-preview-css or @jest-preview/css
• jest-preview-file or @jest-preview/file

They will be dependencies of jest-preview.

Reason: Keep jest-preview more focused on actually PREVIEWING functionality.

We can also consider adding a dedicated package test-preview-server (TODO: Think a better name, I think test-preview is fine) to support other frameworks than Jest in the future.