lint: leave no-undef checking up to TypeScript

[steveluscher]

Problem

• When something is undefined, you get both a TypeScript error and a lint error.
• When something is known to TypeScript (eg. through a @types def) you still get a lint error.

TypeScript ESLint recommends that you leave definedness checking up to TypeScript itself: https://typescript-eslint.io/docs/linting/troubleshooting/#i-get-errors-from-the-no-undef-rule-about-global-variables-not-being-defined-even-though-there-are-no-typescript-errors

This PR has the nice side benefit of making all the red underlines go away in tests, since we in fact do have @types/mocha installed.

Summary of Changes

Disable no-undef in ESLint.

[codecov]

Codecov Report

Merging #24726 (b514d6b) into master (2be4ee3) will not change coverage.
The diff coverage is n/a.

❗ Current head b514d6b differs from pull request most recent head 2d2be6f. Consider uploading reports for the commit 2d2be6f to get more accurate results

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  Coverage    70.0%    70.0%           
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  Lines        2301     2301           
  Branches      325      325           
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  Hits         1613     1613           
  Misses        573      573           
  Partials      115      115