Add support for custom Monaco "completion provider"

GOBL Builder should allow supplying (contextual) autocomplete data, to be used for a custom Monaco Editor [completion provider](https://microsoft.github.io/monaco-editor/playground.html#extending-language-services-completion-provider-example) 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).