JS Loader - Full backwards compat mode

### Problem Statement

We are getting a bit more "aggressive" when it comes to supporting ancient versions of Browsers etc.
I think we should be really mindful about this to not become too strict, Sentry is know it just works, no matter how old your app is.
I am proposing a mode in the loader that is able to automatically fall back older major versions of the SDK in order to make us as compatible as possible.

### Solution Brainstorm

Here is how I think we could do:

Someone imports/loads the loader script
within the script we to load the bundle with the specific version the user selected - if it fails (try/catch)? not sure if we can guard against this tbh - we try to load an older version of the bundle (previous major).

### Product Area

Unknown

The issue does not name files or tests. Start by tracing the JS Loader's versioned bundle loading and determine how a failed load can be detected safely; done means the loader can automatically fall back to an older major SDK version while preserving compatibility for older applications.

Written by the indexing model from the issue text.