fix(build): Add JS polyfills to utils tarball

[lobsterkatie]

UPDATE: This is no longer necessary, and is therefore getting closed.

This is a follow-up to #5023, which introduced a set of tslib-like JS polyfills in the utils package. Because they live in a non-traditional place, they don't automatically get included in the tarball we publish to npm. This fixes that, by adding a utils-specific prepack script to copy them into the build directory. It also adds the resulting tarballs to .gitignore, so this can be tested locally without polluting the working tree.

There's no test to prove that this works, but here is the result of running yarn build:npm in the utils package:

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	18.73 KB (-7% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	58.55 KB (-9.39% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.54 KB (-7% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	52.73 KB (-9.05% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.2 KB (-17.38% 🔽)
@sentry/browser - Webpack (minified)	62.07 KB (-24.05% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.22 KB (-17.43% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	42.9 KB (-10.74% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	24.51 KB (-6.02% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23.09 KB (-5.71% 🔽)

[lobsterkatie]

Because of changes in #5051, this is no longer needed.