Signature help fixes (#950)

* account properly for locs in signature help so the deepest focused item is always the one getting focus

* refactor so that signature help tries to use typed information first for function calls. This will give better results since typed info will have type variables instantiated

* changelog

* clean up signature help

* update tests

Author: zth

CHANGELOG.md

@@ -21,6 +21,7 @@
 - Enhance variant constructor payload completion. https://github.com/rescript-lang/rescript-vscode/pull/946
 - Clean occasional dots from "insert missing fields" code action. https://github.com/rescript-lang/rescript-vscode/pull/948
 - Pick up code actions in incremental compilation. https://github.com/rescript-lang/rescript-vscode/pull/948
+- Various improvements to the signature help functionality. https://github.com/rescript-lang/rescript-vscode/pull/950
 
 ## 1.48.0