Add information about strict_mode for encore

[Philosoft]

I guess it should be both in 5.4 and 6.0, but actually will be useful down to whenever https://github.com/symfony/webpack-encore-bundle/pull/54 was merged

Personally, it took me quite a long time to determine and fix the cause of functional tests failing in CI. I know that stuff is kind of mentioned in config/packages/webpack_encore.yaml, but here are my excuses

• there is certain time gap between you first install webpack, write your functional tests (involved with actual HTML rendering), add some styles/scripts to template and setup CI to run these tests
• this stuff is not mentioned in the docs at all - fgrep -RH strict_mode . gives 0 results