Transition all rx-player scripts to es6 style imports

I realized that in #1309, I broke the demo bundling script and
translated only some of our node.js scripts so they have es6-style
imports instead of commonjs ones.

I fixed the issue and completely transitioned (with two exceptions for now:
eslint and jest configs, but we also could transition them I guess) to
es6-style imports in our scripts to facilitate development (no
need to switch the import style depending on if the JS runs server-side or
client-side).

.github/workflows/checks.yml

@@ -51,7 +51,7 @@ jobs:
     - run: npm install
     # Firefox seems to have issue with integration tests on GitHub actions only
     # TODO to check
-    - run: node tests/integration/run.js --bchromehl
+    - run: node tests/integration/run.mjs --bchromehl
     - run: npm run test:memory
 
   # Windows seems to be a lot less stable for some reason.
@@ -75,7 +75,7 @@ jobs:
   #   - run: npm install
   #   # Firefox seems to have issue with integration tests on GitHub actions only
   #   # TODO to check
-  #   - run: node tests/integration/run.js --bchromehl
+  #   - run: node tests/integration/run.mjs --bchromehl
   #   - run: npm run test:memory
 
   # MacOS seems to be a lot less stable for some reason.

.github/workflows/perfs.yml

@@ -19,4 +19,4 @@ jobs:
     - run: npm install
     - run: export DISPLAY=:99
     - run: sudo Xvfb -ac :99 -screen 0 1280x1024x24 > /dev/null 2>&1 & # optional
-    - run: node tests/performance/run.js
+    - run: node tests/performance/run.mjs