support unpkg way use this plugin

[mlzboy]

I checked the blockly group https://groups.google.com/g/blockly/c/V4RJ9o03qyQ?pli=1 seems this plugin is lack of way under unpkg.

use codepen.io is easy to debug and test, hope add this feature.Most Blockly document for quickstart use unpkg,it's easy to start.

[mark-friedman]

The current version of the lexical block plugin now works with unpkg using the following script tag:

<script src="https://unpkg.com/@mit-app-inventor/blockly-block-lexical-variables/dist/index.js"></script>

You can see some example code in the "works-with-unpkg" branch of the lexical blocks plugin demo.

[mark-friedman]

Feel free to re-open this if you run into any difficulties.