feat(utils): Add function for ensuring input is an array

[lobsterkatie]

In a number of places in our codebase, we have the pattern const something = Array.isArray(somethingElse) ? somethingElse : [ somethingElse ];, where we need something (which might already be an array) to be an array.

This introduces a utility function, arrayify, which does that work for us, mostly because I just got tired of typing it. (As a bonus, it's slightly shorter and therefore might save a few bytes on bundle size.)

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.42 KB (-0.04% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.08 KB (-0.02% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.99 KB (-0.02% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	52.94 KB (-0.03% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.79 KB (-0.01% 🔽)
@sentry/browser - Webpack (minified)	64.33 KB (-0.02% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.81 KB (-0.02% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.72 KB (-0.02% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.91 KB (-0.01% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.28 KB (0%)