Add demo with +/- alternative mutator extensions for if/else, text_join, lists_create_empty, lists_repeat

Problem statement

The block-based mutator UI is an advanced user feature that may not be appropriate for many of our users.

Proposed solution

Build alternative block mutator implementations for the following blocks:

• controls_if
• text_join
• lists_create_empty
• lists_repeat
• Function definitions (argument list)
• Add integration test that imports and exports the serialized blocks, and confirms results.

The alternative mutators should use field_images as + and - buttons to add and remove options. The serialize format should be identical to the existing mutators, allowing developers to switch mutators despite existing saved workspaces in the wild.

This may resolve #1747.

[BeksOmega]

I did some work on this this weekend and I now have a functioning prototype that works for all of the above blocks (except lists_repeat which doesn’t have a mutator).

My questions are: Would you still like a demo of this? And if so, would you want it in this repo, or the samples repo?

You can view my changes here. It still needs cleanup, testing, and docs. I’ll also try and get a demo you can play with working.

[Edit: demo. Tell me if it breaks or you can’t access it.]

[rachel-fenichel]

Blocks refactor: No, avoid the refactor during this one. But I’m interested in a write-up of your conclusions about the mutator APIs when you’re done.

Module/package/etc: Start with a PR with everything in a file that you can include with a script tag. After that (and in a separate PR) follow the example of workspace-search to package it up.