Show function params and return types in completion items for tremor-script

[anupdhml]

In the overall list of possible function completions for tremor-script, param and return type is not included currently. It used to work pre 0.8 (since we used to parse the doc markdown files where that info was structured in a certain way).

Once there’s support for encoding this info in a structured manner as part of tremor function doc, we can modify the language server to use it.