Simplify Namespaces

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

FIxes #620

Purely cosmetic with no functional changes.

• All processing extensions now live under SixLabors.ImageSharp.Processing
• All processors now live under SixLabors.ImageSharp.Processing.Processors.XXX

I've deliberately pushed things like IResampler implementations down into the Processors.XXX namespaces as I wan't to encourage use of the static KnownXXX instances.

I recommend separating out the ImageSharp and ImageSharp.Drawing documentation when we next publish an update to avoid clutter.

[codecov]

Codecov Report

Merging #645 into master will not change coverage.
The diff coverage is 87.5%.

@@           Coverage Diff           @@
##           master     #645   +/-   ##
=======================================
  Coverage   89.32%   89.32%           
=======================================
  Files         893      893           
  Lines       38090    38090           
  Branches     2668     2668           
=======================================
  Hits        34023    34023           
  Misses       3269     3269           
  Partials      798      798

Impacted Files	Coverage Δ
...rp.Tests/Processing/Convolution/DetectEdgesTest.cs	100% <ø> (ø)	⬆️
...rc/ImageSharp/Processing/GaussianBlurExtensions.cs	100% <ø> (ø)
...ng/Processors/Transforms/TransformProcessorBase.cs	100% <ø> (ø)
src/ImageSharp/Processing/SkewExtensions.cs	100% <ø> (ø)
src/ImageSharp/Processing/ResizeExtensions.cs	100% <ø> (ø)
src/ImageSharp.Drawing/Processing/Pen{TPixel}.cs	100% <ø> (ø)
...essing/Processors/Quantization/PaletteQuantizer.cs	100% <ø> (ø)
tests/ImageSharp.Tests/Drawing/DrawPathTests.cs	100% <ø> (ø)	⬆️
...ng/Processors/Convolution/RobertsCrossProcessor.cs	100% <ø> (ø)
...cessing/Processors/Transforms/Lanczos2Resampler.cs	100% <ø> (ø)
... and 279 more

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[antonfirsov]

Agree with your decisions & can't spot anything wrong here.