Optimize System.Text.Encodings.Web using arm64 intrinsics

[BruceForstall]

This item tracks the conversion of the System.Text.Encodings.Web class to use arm64 intrinsics.

Related: #33308

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[tarekgh]

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[BruceForstall]

I'm moving this to Future, as there is no current plan to do the work for .NET 5.