[New Feature]: Create a blacklist of bad-idea namespaces for PRs.

### Description of the new feature/enhancement

Among the recently increasing amounts/percentage of remarkably bad PRs for new packages, some of the most remarkably bad ones use namespaces that are somewhere partway between "Weird" and "Obstructive".

They are generally bad enough that they would cause sizable problems to this repo's filetree structure if they had got merged, thus it'd be easiest for all involved parties if they couldn't be submitted under those namespaces.

Among the current cases:
* #416656.
* #419322.
* #413703.
* #407210.

### Proposed technical implementation details (optional)

If a package's namespace is any of the following (Case-insensitive), throw a `Validation-Manifest-Path-Forbidden` (or similar) error:
* You
* Me
* www
* com
* net
* org
* (A single character)
* Micosoft (and similar typos)
* SampleApp
* SampleProgram
* Hello
* HelloWorld
* Test

Start by locating the manifest namespace validation entry point and review the referenced issues (#416656, #419322, #413703, and #407210) to understand the problematic cases. Done means the specified namespaces are rejected case-insensitively with a validation error while acceptable namespaces continue to pass validation.

Written by the indexing model from the issue text.