Source-aware completions and more completer settings

[krassowski]

References

Small step towards #495 and #236. Fixes #492.

Code changes

• trim spaces in insertText when merging completions (was the source of duplicated completions as Python kernel was returning 'return' and 'return ')
• generalised completion merging code so that plugin in kite or another will be easier in the future
• extend CompletionHandler.ICompletionItemsReply to include source information (ICompletionsReply)
• define ICompletionsSource for source metadata storage
• extend CompletionHandler.ICompletionItem to include source info and others (IExtendedCompletionItem)
• source in IExtendedCompletionItem and ICompletionsReply can be null/undefined at different stages. How to handle this safely for future us?

User-facing changes

• detail is now shown instead of the type by default; if no detail is available, then type or source ("Kernel" for kernel completions) is shown instead.

• exposed settings:
  • caseSensitive: old behaviour retained by default
  • includePerfectMatches: old behaviour retained by default
  • labelExtra: new behaviour - display detail from LSP rather than the type in the completion label

Backwards-incompatible changes

Chores

• linted
• tested
• documented
• changelog entry

[krassowski]

I am not 100% sure if the new behaviour with detail should be enabled as default. It seems to work fine, but there can be this awkward delay visible on the GIF when ggplot2 and geom_ functions are being typed, when the completion items are being resolved (i.e. the detail gets fetched from the server).