Signature help should be triggered when entering placeholders following trigger chars #164310
Description
When a completion provider returns a snippet like atan2(${1:y}, ${2:x}), the user should see signature help when editing the parameters.
VSCode could trigger signature help automatically when entering these placeholders (given that ( and , are trigger characters) but does not do so.
Other editors do this and provide noticeably a better signature-help flow with the same language servers (e.g. Neovim+nvim-cmp+lsp_signature.nvim).
This is a near-dupe of #78806 which is locked, I believe the resolution wasn't right.
It was suggested that providers should attach a command to the completion item to trigger signature help. However this doesn't really achieve the goal: will only fire when first entering the first parameter placeholder for the first time, it will not trigger when tabbing to other parameters, or when returning to the first one.
Moreover that idea requires changes to LSP, various language servers and other plugins, which means it may be a long time until it is available to users (3 years so far), and doesn't seem necessary.