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

[czerwinskilukasz1]

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

[czerwinskilukasz1]

Blocked on #45 (type semantics)