Closes #31.
Relates to #23. ESLint should tell us if we try to use features from ES7+.
Relates to #39.

Changes

• Adds a package.json file to support npm dependencies
• Adds ESLint as a dev dependency from npm
• Adds an ESLint configuration file with just the basic recommended rules for now. It applies to all JavaScript files in static/js except for minified ones like html.min.js.
• Adds an npm script so we can run the linter with npm run lint
• Adds a GitHub Action to run ESLint for pull requests to the main branch
• Adds linting instructions to README.md
• Fixes a few bad practices ESLint already caught (missing variable declarations)

Notes

For now I'm just informing ESLint about global variables like lucide and queryStructure at the top of the files that use them.