adds noLabelVar to linter

[gobeli]

Adds no-label-var check for #94
Maybe also check for BROWSER / NODE / JEST - Variables?

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[sebmck]

Perfect! Thank you!

Maybe also check for BROWSER / NODE / JEST - Variables?

We should probably move that out of the specific undeclaredVariables rule into some logic in isGlobal, and add some way of specifying an environment.