[Framework] Read env var SYMFONY_IDE by default for framework.ide

[94noni]

Fix #16232
friendly ping @GromNaN (very useful :))

[GromNaN]

Thank you @94noni. Could you update the default value also.

[94noni]

@GromNaN please see bb2b5fb if it is what you meant, thanks

[javiereguiluz]

Thanks Antoine.

[javiereguiluz]

Please note that while merging this, I removed the entire config example that used an env var, because the good thing of this feature is that you only need to configure the SYMFONY_IDE env var and do nothing else.