[WIP] Switch unit tests to use vitest

To extrapolate on #1444, this PR shows how it would look like to switch
from relying on Jest to rely on vitest for our unit tests.

The end goal being to simplify our codebase by relying on a single
testing framework.

---

I only done so for testing files in the `src/compat` directory in this
demo. It can be tested right now by calling `npm run test:unit:vitest`.

---

Sadly for now, we are still forced to rely on a JSDom-ed Node.js
environment for unit tests and a browser environment for integration
tests - meaning we have very different configs for both.

This is because we want to mock imported files in unit tests - something
that is not possible for now in browser environment through vitest
(though vitest/5765 seems to have
been merged very recently so maybe we could rely on the browser for both
soon), yet we want to replicate as much as a real browser as possible in
our integration tests (because we're also testing that media playback on the
tested browsers goes as expected).

Author: peaBerberian

.eslintrc.js

@@ -198,9 +198,7 @@ module.exports = {
     "import/no-deprecated": "error",
     "import/no-extraneous-dependencies": [
       "error",
-      {
-        devDependencies: false,
-      },
+      { devDependencies: ["**/*.test.ts", "**/__tests__/**"] },
     ],
     "import/no-internal-modules": "off",
     "import/order": [

FILES.md

@@ -183,9 +183,7 @@ The rx-player contains multiple type of tests:
 - unit tests: test specific parts of the code. The main goal here is to check the
   implementation of smaller units of code.
 
-  They are written alongside the code, in `__tests__` directories. All its configuration
-  can be found at the root of the project, in `jest.config.js` (we use the jest library
-  for unit tests).
+  They are written alongside the code, in `__tests__` directories.
 
 - memory tests: test the memory usage of the player.