chore: switch to pnpm monorepo

[mcous]

Overview

In order to release core as its own package, without @testing-library/dom included, this repo needs to become a monorepo to hold both packages

This PR switches over to a pnpm monorepo with a single package - the existing @testing-library/svelte - to get CI and the release pipeline stable before attempting the package split

Change log

• Move src to packages/svelte/src
• Split package.json into dev at repo root and prod in packages/svelte/src
• Switch tsconfig over to project references
• Switch publishing to multi-semantic-release
• Rework Svelte/Node test matrix environments to work with pnpm
• Update CI configs to use pnpm
• Update contributing docs to use pnpm
• Remove a few things that became unnecessary or stopped working