feat(build): Vendor polyfills injected during build

[lobsterkatie]

Both Rollup and Sucrase add polyfills (similar to those in tslib) during the build process, to down-compile newer language features and help ESM and CJS modules play nicely together. Unlike with tslib, however, there's no option equivalent to tsc's importHelpers option, which allows those polyfills to be imports from one central place rather than copies of the full code for each necessary polyfill in every file that uses them. Having all that repeated code is a surefire way to wreck our bundle-size progress, so as part of the switch to using those tools, we will replicate the behavior of importHelpers in the context of our new sucrase/rollup builds.

This is the first of two PRs accomplishing that change. In this one, the polyfills we'll use have been added to the repo. In the next one, a rollup plugin will be added to replace the duplicated injected code with import statements, which will then import the polyfills this PR adds. Originally this was one PR, but it has been split into two for easier reviewing.

Notes:

• The polyfills themselves have been added to @sentry/utils, in a separate folder under src/. Various discussions have been had about where they should live, and they have moved around as this PR evolved. Without going into too much boring detail about about the exact pros and cons of each location, suffice it to say that my first instinct was to put them in utils, and as I worked on other options, each one of the reasons why I didn't actually do that to begin with (they weren't in TS, they weren't originally ours, I didn't want to pollute the @sentry/utils namespace) got solved, and in the end, putting them in utils was the simplest and most logical option.
• The added polyfills are copies of the code injected by Sucrase or Rollup, translated into TS and in some cases modified in order to make them less verbose. Since some treeshaking algorithms can only work file by file, each one is in its own file. Polyfills which were modified have had tests added, both to ensure that they do the same thing as the originals, and to ensure that what they do is actually correct.
• As we're not the original authors, attribution has been given within each file to the original source. Further, since the MIT license under which both Rollup and Sucrase are distributed requires duplication of the license blurb, it has been added to the README in the polyfills' folder.
• Because we don't want these to become part of our public API, their code is not exported by index.ts, meaning they are not importable directly from @sentry/utils and won't come up under intellisense code completions and the like. When our code imports them, it will import from @sentry/utils/cjs/buildPolyfills or @sentry/utils/esm/buildPolyfills as appropriate.
• Though not every polyfill Rollup and Sucrase can inject ends up being used in our built code, they are all included just to cover our bases. That said, in the interest of time, only the ones we use or are likely to use have had tests written for them.
• One of the polyfills we do use is the optional chain polyfill. Typing it is tricky, because it takes a variable number of arguments, whose types are effectively [ A, B, C, B, C, B, C,... ] (one type for the first argument, and then alternating types for the rest). Because it's not going to be used by the public, the typing in the polyfill itself uses the generic unknown. For tests it was easier, because only the test cases needed to be typed, and could have a slightly modified structure: [ A, [ B, C ][] ], which then gets flattened again before being passed to the optional chain polyfill. In order to be able to use Array.prototype.flat in tests run on older versions of Node, it itself has been polyfilled, and our testing tsconfig target ramped back down when running tests under those old versions.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	18.76 KB (-6.87% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	58.58 KB (-9.35% 🔽)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.55 KB (-6.97% 🔽)
@sentry/browser - ES6 CDN Bundle (minified)	52.76 KB (-9% 🔽)
@sentry/browser - Webpack (gzipped + minified)	19.21 KB (-17.35% 🔽)
@sentry/browser - Webpack (minified)	62.12 KB (-23.99% 🔽)
@sentry/react - Webpack (gzipped + minified)	19.23 KB (-17.39% 🔽)
@sentry/nextjs Client - Webpack (gzipped + minified)	42.75 KB (-11.04% 🔽)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	24.42 KB (-6.34% 🔽)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	23 KB (-6.08% 🔽)

[Lms24]

Thanks for the detailed explanation the PR description. I think we've gone back and forth about it and if putting these files into utils is the best solution, I'm totally okay with it.