Always suggest cabal-version field for the first line of a cabal file #5115

We know that the first line of the cabal file should always be cabal-version: ....

Therefore we should consider this when generating completions for .cabal files.

This will be a little bit of a hacky solution, as we add add a check for whether we are in the first line of the cabal file when deciding the context for completions.

Implementation suggestion:

• Extend FieldContext type in Completion.Types.hs in the cabal plugin by a Context which denotes that we are in the first line of the cabal file.
• In findCursorContext in Completion.hs add a case for the cursor being in at the beginning of the first line of the file and then return this new FieldContext.
• Add a completion for this context in the contextToCompleter function.