build: Turn unused vars from TS error into eslint error

[lforst]

Unused variables prevent us from building which is quite annoying when just testing things out and brainstorming solutions.

This PR turns the "unused var/argument" error from a typescript error into a proper eslint error, which will allow us to still build, even when there are unused variables. CI will now instead fail on the lint step, which is an added benefit because it is generally faster to retry than the build step.

[Lms24]

Just yesterday, I was annoyed by this while fighting Replay tests... So i'd vote to do this, as long as we're still failing the lint job in CI 👍

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.84 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	61.46 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.62 KB (+0.02% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	55 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.38 KB (0%)
@sentry/browser - Webpack (minified)	66.55 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.4 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.63 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.82 KB (+0.02% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.25 KB (-0.01% 🔽)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	43.13 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	38.33 KB (0%)

[mydea]

nice! I took the freedom and renamed the scope from ci to build, as I guess it actually applies to that :) will make debugging less annoying 👍