[UX][Bug] Avoid considering "false" as a truthy value for "synchronize_package_json"

[Crovitche-1623]

The current command composer config "extra.symfony/flex.synchronize_package.json" false write a string containing "false":

e.g.

{
    {
        "symfony/flex": {
            "synchronize_package_json": "false",
        }
    }
}

It does not skip the package.json as expected so I suggest to add the filter_var, so for al those use the previous "wrong" command, it'll work as expected now.

ping @smnandre and @Kocal

related: symfony/ux#3091

[Crovitche-1623]

Note: the CI step for CS fails on code that I didn't wrote.