Add support for custom Monaco "completion provider"

[dstotijn]

GOBL Builder should allow supplying (contextual) autocomplete data, to be used for a custom Monaco Editor completion provider for supplying autocomplete suggestions for well-known JSON properties based on their JSON Schema definitions. E.g. for https://gobl.org/draft-0/org/party, the parent project should be able to provide a callback to provide a list of customers (based on the context/current position of the cursor in the document).