Add list-setter shadow blocks at top of Lists flyout

[tracygardner]

Summary

• switched the Lists toolbox category to use a custom flyout callback (custom: "LIST")
• added four set [list] to ... blocks at the top of the Lists flyout:
  • set list to empty list
  • set list to a list of 2 numbers (shadowed)
  • set list to a list of 2 text values (shadowed)
  • set list to a list of 2 colours (shadowed)
• preserved existing standard list blocks below these new entries

Implementation details

• registered a new LIST toolbox category callback in main/blocklyinit.js
• built flyout XML programmatically so list setter blocks include the required nested list shadows
• list variable names default to incremented listN style based on existing variables in the workspace

Files changed

• toolbox.js
• main/blocklyinit.js

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Codex Task