[v8] Merge `@sentry/utils` into `@sentry/core`

[mydea]

In v8, let's merge utils into core, so all shared, runtime agonstic things are in utils, while runtime specific things should be moved out of utils into either browser or server-runtimes packages.

If we have things that are e.g. browser specific that are needed by e.g. replay or feedback packages, we need to see what we do with them. But either we create a new e.g. browser-utils package for this (if needed) or, if it's not a lot, we just duplicate stuff a bit - we'll need to see case-by-case if this is necessary.

### Tasks
- [ ] https://github.com/getsentry/sentry-javascript/pull/11381
- [ ] https://github.com/getsentry/sentry-javascript/pull/11451
- [ ] Move rest of imports into @sentry/core or @sentry/node
- [ ] Delete utils package

[AbhiPrasad]

Given we have moved all browser related code into the browser utils package, this can be considered done.

We can now review on a more case-by-case basis if we should move functions from utils