This allows a file other than LocalSettings.php to be used as the primary
settings file by setting the MW_CONFIG_FILE environment variable.
This also allows the primary settings file to use YAML or JSON format.
Using static configuration files should be the default in the future.
However, YAML files in the document root could easily be exposed to the
public. Better not to encourage that, and require them to be enabled
explicitly and loaded from a different place.
Bug: T294750
Change-Id: I7747f83481cb05a6d05f819be652259951183819
