Flatten React-DnD Directory Structure

[darthtrevino]

The scope of this PR has changed quite a bit. Separate packages helped to clarify top-level concerns, but practically this is problematic. It would bifurcate whether library-authors would use @react-dnd/core or react-dnd, and resolving context types would become a nightmare. Instead this just reorganizes the existing react-dnd package around these high-level concerns and flattens the directory structure a bit.

The primary driver of this refactor is separating the hooks-based API from the decorators-based API. The decorators pattern is a little flakier, but has been around much longer. It requires some dependencies that the hooks API does not. They are basically independent surface areas achieving the same thing. We will still publish react-dnd with both the hooks and decorator API, but these APIs will have the ability to be consumed independently of each other.

New Packages:
### Public Facing
@react-dnd/hooks - The Hooks API
@react-dnd/decorators - The Decorators API
@react-dnd/core - Context Types, Provider API (required peer of hooks/decorators)
### Internal
@react-dnd/internal - Common internal components (not intended for client usage)
@react-dnd/types (to eliminate dep cycle between core/internals. Re-experted by core)

[codecov]

Codecov Report

Merging #3038 (d42f9c1) into main (120a6bf) will increase coverage by 4.38%.
The diff coverage is 31.93%.

@@            Coverage Diff             @@
##             main    #3038      +/-   ##
==========================================
+ Coverage   40.81%   45.19%   +4.38%     
==========================================
  Files         253      248       -5     
  Lines        3788     3790       +2     
  Branches      768      770       +2     
==========================================
+ Hits         1546     1713     +167     
+ Misses       2241     2076     -165     
  Partials        1        1              

Impacted Files	Coverage Δ
...ckages/dnd-core/src/classes/DragDropMonitorImpl.ts	96.22% <ø> (ø)
...ckages/dnd-core/src/classes/HandlerRegistryImpl.ts	84.53% <ø> (ø)
packages/react-dnd/src/core/DragPreviewImage.tsx	7.14% <ø> (ø)
packages/react-dnd/src/decorators/DragLayer.tsx	20.96% <ø> (ø)
packages/react-dnd/src/decorators/DragSource.ts	87.50% <ø> (ø)
packages/react-dnd/src/decorators/DropTarget.ts	100.00% <ø> (ø)
...es/react-dnd/src/decorators/createSourceFactory.ts	20.93% <ø> (ø)
...es/react-dnd/src/decorators/createTargetFactory.ts	45.45% <ø> (ø)
...kages/react-dnd/src/decorators/decorateHandler.tsx	86.90% <ø> (ø)
packages/react-dnd/src/decorators/disposables.ts	52.94% <ø> (ø)
... and 26 more