Issue #3616878: Fix eslint and gate it

First step of the lint burn-down plan in #3616878: eslint is now clean (both no-jquery/no-val errors fixed by hand, everything else eslint --fix corrected) and _ESLINT_ALLOW_FAILURE is set to 0 so it stays clean.

phpstan, phpcs, and stylelint are separate follow-up commits per the issue's own plan — kept apart so each tool's diff stays reviewable on its own.