fix(build): Prevent rollup from adding [Symbol.toStringTag]: 'Module' to CJS files (#6043)

As of version 2.69.0, setting `output.generatedCode` to `'es2015'` (as we do) causes Rollup to [add `[Symbol.toStringTag]: 'Module'` to generated CJS files](rollup/rollup#4378 (comment)). Though this is valid ES6, it causes Jest to be unable to mock our generated packages.

Though [a PR](jestjs/jest#13513) has been opened to fix this, the change almost certainly won't be backported, so anyone using Jest 29.2.2 or under will run into [this problem](#5994) if they try to mock `@sentry/xxx` 7.16. (The relevant change was introduced in #5951, when we (semi-)accidentally upgraded Rollup from 2.67.1 to 2.78.0, and was first included in version 7.16.) 

This PR prevents the new Rollup behavior, since it has no known benefit. (The [Rollup docs](https://rollupjs.org/guide/en/#outputgeneratedcode) say that presence of the `'Module'` toStringTag "is used for feature detection in certain libraries and frameworks," but not having it hasn't seemed to hurt us so far.)

Author: lobsterkatie

rollup/npmHelpers.js

@@ -44,8 +44,14 @@ export function makeBaseNPMConfig(options = {}) {
       // output individual files rather than one big bundle
       preserveModules: true,
 
-      // any wrappers or helper functions generated by rollup can use ES6 features
-      generatedCode: 'es2015',
+      // Allow wrappers or helper functions generated by rollup to use any ES6 features except symbols. (Symbols in
+      // general are fine, but the `[Symbol.toStringTag]: 'Module'` which Rollup adds alongside `__esModule:
+      // true` in CJS modules makes it so that Jest <= 29.2.2 crashes when trying to mock generated `@sentry/xxx`
+      // packages. See https://github.com/getsentry/sentry-javascript/pull/6043.)
+      generatedCode: {
+        preset: 'es2015',
+        symbols: false,
+      },
 
       // don't add `"use strict"` to the top of cjs files
       strict: false,