Add beginner add and delete blocks to Lists toolbox

[tracygardner]

Motivation

• Provide beginner "add [item] to [list]" and delete entry in the Lists toolbox so users can append items without composing a full lists_setIndex configuration.

Description

• Inserted a preconfigured lists_setIndex block (with MODE: "INSERT" and WHERE: "LAST") immediately before the existing lists_create_empty entry in toolbox.js to act as the add-item block.

Testing

• Ran npm run build which completed successfully; and ran the dev server plus a Playwright script to open the app and capture the Lists toolbox, both of which succeeded.

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Codex Task