eslint: Re-enable no-undef for TypeScript.
eslint: Re-enable no-undef for TypeScript.

@typescript-eslint/recommended disables no-undef on the theory that
TypeScript knows which globals are in scope.  However, some TypeScript
definitions include declarations for globals that may or may not
actually be available depending on the environment.  So we will need
to prohibit some globals that TypeScript knows about, and enforce that
they are properly imported.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

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