Adds base ESLint configuration to the wpcomsh plugin.

[zinigor]

Fixes ESLint errors when used with base JS ESlint configuration for the Jetpack Monorepo.

Proposed changes:

• Brings in ESLint base config to wpcomsh.
• Fixes ESlint warnings and errors.
• Adds some files to ESLint ignore file.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Go to '..'

[github-actions]

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Wpcomsh plugin:

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[anomiex]

🎉 Thanks!

A few things inline, although some of them probably don't need any action.

[anomiex]

Eventually we probably should lint this, but OK with me if we do that as a followup.

[anomiex]

If we get around to setting up a webpack build for the scripts here in some future PR, we could also look at whether there's an npm package we could use instead of copy-pasting these in.

The spin.js looks like it's probably an old version of https://www.npmjs.com/package/spin.js.

[anomiex]

LGTM other than the one remaining lint error.