Add a GitHub Action that runs the formatter

[maribethb]

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Component

entire repo

Problem

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Request

After #1946 is submitted, we'll have a new top-level script that can automatically format the entire repository. On every pull request, we should run npm run format:check which will run the formatter in a mode that reports if any file needs formatting still. We have a similar action in core blockly here. We should add a similar job to our existing build/lint action in blockly-samples.

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[sdthaker]

Hi there! Can you please assign this to me? I'd like to work on this issue. Currently working on # 1978, will get to this one once I'm done with that. Thanks!