[PCC-1371] Fix preview and publish to local targets

[a11rew]

Overview

#269 fixed a regression in core causing preview and publish to not work with local (localhost://) targets.

This PR builds on that hotfix by removing the problematic code path that made the issue even possible and adds tests to prevent this from happening again.

Changes

core

• Removed base url construction for redirects. All redirects are now to relative paths, which is valid in the HTTP spec. Trying to determine the request's absolute url in PantheonAPI is the problematic code path talked about above, it introduces unnecessary complexity we don't have to deal with when we just leave that to the HTTP client.
• Removed app router specific code from core, essentially reverting it to its state in f5e47b3. This was to make it easy to validate the core PantheonAPI routing logic in tests without dealing with brower/Next.js constructs like Response and Headers
• Added tests to validate routing logic

react-sdk

• Moved changes added to core to support app router here
• Added tests to validate the NextPantheonAPI here correctly handles requests and responses in both pages and app routing

vue-sdk

• Removed app router support specific changes added here

Other changes

• Updated build process in packages so sourcemaps are output separately instead of inline with application code which unnecessarily increases our package sizes.

[a11rew]

Merged this into the core tests as it tested routing logic specific to PantheonAPI in core

[kevinstubbs]

Add changeset please.