Add/configure eslint and husky

[mbklein]

• Add eslint as an explicit dependency
• Add eslintConfig section to package.json
• Use husky to run eslint & prettier before commit
• Fix all eslint complaints in the existing codebase

The vast majority of existing complaints from the linter were about variables that were either declared/assigned but unused, or undeclared (i.e., missing let or const before first assignment).

You can always use the --no-verify flag on a git commit to skip the pre-commit hook if you're confident and willing to live with the consequences. 😄