Prioritise completion suggestion of required fields

[VeryMilkyJoe]

Related to #3664.

When suggesting fields in the top level or in stanzas of a cabal file, it would be nice to prioritise suggestion of required fields over optional ones.
Additionally, it would be nice to label required fields as such using the details field of the CompletionItem.

• In order to implement this, the data structure storing the keywords and their completers needs to be updated with some way of marking the required fields.
• Then, the weightedConstantCompleter function can be used to prioritse the suggestion accordingly.
• In order to add a label, the CompleterData probably needs to carry the information whether the suggested field is required.