Gulp eslint

[benjaminpjones]

Addresses Gulp task in #1536

Proposed Changes

Add eslint to gulp using the gulp-eslint7 package.

Notes

• I did not re-implement debouncing as it seems that gulp does some debounce handling on its own (using the delay option, whose default is 200 ms). I verified this by updating 100 files at once (eslint --fix and git checkout src), and gulp seems to do okay. Let me know if there is a scenario I am missing.
• gulp-eslint (without the 7) is in a weird place right now. It is not actively maintained, but there are some forks that are gaining traction. gulp-eslint7 seems to be the best fit for this project, since it uses ESLint v7.32. gulp-eslint-new also exists and may be the project to use going forward, but it uses ESLint v8.0, which breaks some rules.

[anoek]

Awesome, thanks for researching that and making it happen!