Add the `sendDefaultPii` option to the JS SDKs. This patch only adds the flag but functionality will be introduced in future PRs

…#5344)

Add the check if `sendDefaultPii` is set to `true` before adding the `user_id` when populating DSC.
Additionally, add some tests to check for the changed behaviour.
Conforms with Sentry Dev Spec for handling sensitive data in DSC: https://develop.sentry.dev/sdk/performance/dynamic-sampling-context/#handling-sensitive-data-and-pii

Add the sourcemaps upload script to the tarball, which required the following changes:
* Add a package-specific `prepack.ts` script which is invoked by the main `prepack.ts` script. This is similar to how we do it with the Gatsby SDK where - just as with Remix - we need to copy over additional files to the `build` directory that are not part of our other packages. The newly added files are:
  * `scripts/sentry-upload-sourcemaps.js`
  * `scripts/createRelease.js`
* Add a package-specific `.npmignore` which extends the root level `.npmignore` file to not ignore the `scripts` directory in the `build` directory.

Co-authored-by: Lukas Stracke <[email protected]>

This patch temporarily removes the `user_id` and `transaction` (name) fields from the dynamic sampling context, meaning they will no longer propagated with outgoing requests via the baggage Http header or sent to sentry via the `trace` envelope header.

We're taking this temporary measure to ensure that for the moment PII is not sent to third parties.