Allow for configuring private ontologies

[ajuvercr]

Please add a way to configure alternative ontologies that also:

1. include prefix completions
2. include property and class completions when that ontology is added

This also paves the way to download all LOV ontologies and have faster (offline) completions.

This requires:

1. a way to configure the location where ontology files are stored. This might be global or local for a project (toml file)

Prefix completions can be accomplished with the ontology's preferred prefix.

Complexity comes when people don't use the preferred prefixes :/
As it makes sense that when a user goes to definition to an ontology file the name of the file is preferred ontology.
Can we mark this as an error? :D