ref(build): Split up rollup config code (#4950)

As part of the new build process, a fair amount of new rollup-related code is going to be added. To keep things from getting too unwieldy, this splits the existing code up into modules. It also makes two other small changes, one for consistency and one to differentiate the current rollup code (which is for building bundles) from the future rollup code (which will be for building npm packages):

- All plugins are now generated through factory functions (`makeXXXPlugin`).
- Both the `makeConfigVariants` function and the individual `rollup.config.js` files have been renamed to make it clear they're for creating bundles.

For now all of the resulting modules live in a `rollup` folder at the top level of the repo. In the long run, these would be good candidates to go into an `@sentry-internal/dev-utils` package.

Author: lobsterkatie

packages/wasm/rollup.bundle.config.js

@@ -3,7 +3,7 @@ import { makeBaseBundleConfig, makeBundleConfigVariants } from '../../rollup/ind
 const baseBundleConfig = makeBaseBundleConfig({
   input: 'src/index.ts',
   isAddOn: true,
-  jsVersion: 'es6',
+  jsVersion: 'es5',
   licenseTitle: '@sentry/wasm',
   outputFileBase: 'bundles/wasm',
 });