Is it possible to replace Excel's built-in VBA Editor or IDE with a modern one such as VS Code or VS Studio?
As a C# developer, I prefer the Visual Studio IDE. Excel's VBA Editor/IDE is very dated by comparison. It lacks the sophisticated features found in even the most basic IDEs, such as column selection, jump to references, and useful keyboard shortcuts. As an example, there's no keyboard shortcut for terminating a debugging session.
While it does support many useful features such as suggestions, syntax checking and syntax coloring, they don't work well. It still lacks anything like intellisense. The syntax checking is very obstructive and limited to the most obvious cases. The syntax coloring is limited to keywords and comments. It's autocompletion only works in limited scenarios.
You might consider using VB.NET instead of VBA for your Excel development. Excel-DNA (which I develop) is an open source framework for integrating .NET in Excel, which tries to make the VBA to .NET conversion as easy as possible.
Then you can use any editor for the VB.NET (or C#) code, including the Visual Studio Express editions or SharpDevelop.