Remove ReactWindowsCore projects

[JunielKatarn]

Source folder and projects ReactWindowsCore have become redundant, and only increase project structure complexity:

• A shared (VCXITEMS) project already exists and is used for its current purpose: Shared\Shared.vcxitems.
• Since Desktop and Universal builds take place in different solutions, there is currently no shared static library that would reduce build times.
• Since Desktop and Universal have different settings for Hermes and V8, two separate static libs had been created: ReactWindowsCore.vcxproj and ReactWindowsCore-Desktop.vcxproj.
  These were directly consumed ONLY by their corresponding Desktop and Universal projects, thus adding no build benefit over a shared items project (i.e. Shared.vcxitems).

This change simplifies the solution structure moving the shared source files into their intended project.

Note: This change seems rather large, but it's only because sources are moved from the ReactWindowsCore subdirectory to Shared.

Microsoft Reviewers: Open in CodeFlow

[JunielKatarn]

@vmoroz I have scaled down the size of this change:

• Removed ONLY the ReactWindowsCore project, but left the sources in place so the change is not deemed as "too big". Will revisit moving the sources after we start working on 0.63.
• Reverted the Desktop root namespace renaming. Will submit it on a separate change.

C.C. @asklar

[vmoroz]

revert?

[NikoAri]

[jonthysell]

@JunielKatarn Please make a corresponding update to the docs repo for native modules setup to indicate that this project is no longer present nor necessary. If this is getting backported to 62, you'll want to make these changes now, before the website releases the 62 docs. Otherwise, you want to wait until after the website snaps to 62, and then update the docs.

[JunielKatarn]

@jonthysell certainly. I'll address this today.

[NickGerleman]

Removing the Backport to 0.62 label, since we need to rename it as part of the deprecation in favor of Request Backport which was already added to this PR. Triage of this item should still happen soon.

[Khalef1]

We discussed this in triage and decided not to backport the change. It's nice to have but will also cause a breaking change to the template project