[vscode] Provide error diagnostics via Language Server

[czerwinskilukasz1]

We would like to make AskScript developer-friendly, therefore we need developer tools in VS Code, such as providing diagnostics for typing errors, similar to what VSCode provides for Typescript:

Info on integration with VS Code can be found here: https://code.visualstudio.com/api/language-extensions/programmatic-language-features#provide-diagnostics