Expose visitor through advanced namespace.

[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

• Adds new AcceptVisitor extension method to the Advanced namespace and redirects all calls to that method.

[antonfirsov]

One remark, otherwise looks good!

[codecov]

Codecov Report

Merging #1010 into master will increase coverage by 0.11%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master    #1010      +/-   ##
==========================================
+ Coverage   89.74%   89.86%   +0.11%     
==========================================
  Files        1098     1098              
  Lines       48828    48832       +4     
  Branches     3435     3435              
==========================================
+ Hits        43821    43881      +60     
+ Misses       4254     4249       -5     
+ Partials      753      702      -51

Impacted Files	Coverage Δ
.../Processing/Processors/ImageProcessorExtensions.cs	100% <ø> (ø)	⬆️
...rocessing/Processors/Drawing/DrawImageProcessor.cs	100% <ø> (ø)	⬆️
src/ImageSharp/Image{TPixel}.cs	89.65% <0%> (ø)	⬆️
src/ImageSharp/Advanced/AdvancedImageExtensions.cs	93.33% <100%> (+0.47%)	⬆️
src/ImageSharp/Image.cs	93.54% <100%> (-0.21%)	⬇️
...harp/Processing/Extensions/ProcessingExtensions.cs	100% <100%> (ø)	⬆️
...files/ICC/DataWriter/IccDataWriter.TagDataEntry.cs	77.14% <0%> (ø)	⬆️
...ils/ReferenceImplementations.StandardIntegerDCT.cs	93.68% <0%> (ø)	⬆️
src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs	94.36% <0%> (+0.17%)	⬆️
src/ImageSharp/Formats/Png/PngDecoderCore.cs	85.65% <0%> (+0.21%)	⬆️
... and 25 more

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