Fix Babel issue when running Storybook.

Bug Description

A number of engineers have been seeing the following output from Babel in the terminal when attempting to run npm run storybook or processes that invoke this under the hood (e.g. npm run test:visualtest):

The “loose” option must be the same for @babel/plugin-proposal-class-properties, @babel/plugin-proposal-private-methods and @babel/plugin-proposal-private-property-in-object (when they are enabled): you can silence this warning by explicitly adding [“@babel/plugin-proposal-private-property-in-object”, { “loose”: true }] to the “plugins” section of your Babel config. Though the “loose” option was set to “false” in your @babel/preset-env config, it will not be used for @babel/plugin-proposal-private-property-in-object since the “loose” mode option was set to “true” for @babel/plugin-proposal-private-methods.

We should investigate the causes and determine an appropriate fix. It could be that be changing our Babel plugins as suggested in the warning fixes the issue, or perhaps we need to address some sort of dependency version mismatch.

Steps to reproduce

1. Using the latest develop branch, run npm run storybook, and see the output in the terminal.
2. Removing node_modules and re-running npm install did not fix the issue. If you are not seeing the issue, you could try doing this.

Additional Context

• OS: Mac OS Big Sur 11.2
• Node version: v14.15.4 (using NVM)

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Acceptance criteria

• No babel warnings should be output when building Storybook

Implementation Brief

• Merge https://github.com/google/site-kit-wp/pull/3595

Test Coverage

• N/A

Visual Regression Changes

• No changes expected

QA Brief

• Running the Storybook build should not generate the aforementioned Babel warnings
• Storybook should work as before

Changelog entry

• N/A

[johnPhillips]

@wpdarren I think this is likely to be QA:eng, but will let Evan or Matt make the call 👍

[danielgent]

QA ✅

I no longer see all the babels warnings when running storybook 👍