fix(optimizer): add missing dependency node-fetch

[merceyz]

What's the problem this PR addresses?

In #835 @ampproject/toolbox-optimizer started using node-fetch but didn't declare it as a dependency.

How did you fix it?

Add missing dependency node-fetch

[sebastianbenz]

Sorry about missing this again. Is there a simple way to check this via linter?

[merceyz]

Sorry about missing this again

No worries 👍

Is there a simple way to check this via linter?

Yes, there is a ESLint plugin that can handle this.
https://github.com/benmosher/eslint-plugin-import
https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/no-extraneous-dependencies.md

[sebastianbenz]

Thanks! Unfortunately, it looks like it doesn't play well with how lerna handles dependencies.

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