Show function signature while providing the arguments

[mfronczyk]

Is your feature request related to a problem? Please describe.

When adding a function call, the extension suggests a list of functions with signatures, but when a function is selected and you start providing the parameters, the signature is not visible anymore and you need to bring the signature back sometimes for more complex functions.

Describe the solution you'd like

It'd be nice to keep the tooltip with the function signature open till you finish the call and make the current parameter bold there. It seems that the VS code extension for Idris has such feature, so maybe it'd be possible to add it here also?

Please see the Idris solution here - https://github.com/zjhmale/vscode-idris/commit/fd424291c73a03b61822603df65bf08bc945f00b