Trigger parameter hints on accepting a function-like completion item

[a7medev]

Description

We're working on implementing signature help in SourceKit-LSP (swiftlang/sourcekit-lsp#2250) and we faced a problem in VS Code where signature help wasn't triggered after accepting a completion item for a function, method, subscript, initializer, or enum case with associated values that would've normally had signature help if the user manually typed the ( after the signature name.

Here's a demo of the issue with a development version of SourceKit-LSP:

before.mp4

To fix this, we added a provideCompletionItem middleware that inspects completion items that might have signature help (functions, methods (includes subscripts), initializers, and enum cases) and injects an editor.action.triggerParameterHints command (if no other command was present). This results in signature help working pretty well with completion as shown in the video below.

after.mp4

Tasks

• Required tests have been written
• Documentation has been updated (N/A)
• Added an entry to CHANGELOG.md if applicable