[vscode] Provide Function and Method Signatures Tooltips via Language Server

We would like to make AskScript developer-friendly, therefore we need developer tools in VS Code, such as providing function and method signatures tooltips, similar to what Typescript has:


Info on integration with VS Code can be found here: https://code.visualstudio.com/api/language-extensions/programmatic-language-features#help-with-function-and-method-signatures

Start with the linked VS Code language-features documentation and locate the repository's language-server integration; no source file or test is named in the issue. Define the signature-help behavior for AskQL functions and methods, then verify in VS Code that signatures appear at relevant call sites.

Written by the indexing model from the issue text.