Improve linting to add @typescript-eslint/no-shadow

[johnbley]

Based on @mhennoch discovering a logic bug in #1868 which was caused by shadowing, we should improve our linting. I took a stab at adding

  "no-shadow": "off",
  "@typescript-eslint/no-shadow": ["warn"],

but a few complications arose which made this more than an easy 15-minute PR:

• @typescript-eslint/no-shadow requires updating gts from 2.0.2 (latest is 3.0.1), which brings in many other changes and would require additional lint:fix runs and/or tweaking rule sets.
• @typescript-eslint/no-shadow does appear to have some false positives, namely in testing code where it is common to reuse variable names in multiple it blocks. I would recommend the rule being warn for now based on that.