Add support for multiple code files in Blocks

[alex-kulcsar]

Is your feature request related to a problem? Please describe.
Low-powered machines, in particular, can get bogged down when rendering the workspace for large Blocks games.

Describe the solution you'd like
We already support multiple code files in JavaScript and Python. Why not extend this capability to Blocks? You could do an alternate presentation ... like tabs in an Excel workbook ... but we already have an established UI for the typing languages. Exposing that UI in Blocks mode also allows extensions to be deleted, reloaded, and updated without needing to switch to one of the typing languages.

If the file manager seems too distracting for Blocks mode, then hide it until activated elsewhere in the UI.

Describe alternatives you've considered
Splitting Blocks code into separate projects and then importing them as extensions works as an alternative, but it's clunky.

Additional context
Aside from helping low-powered machines to handle large and complex games, splitting Blocks code into multiple workspaces can help organize code and, potentially, teach users about modularity and code reuse.