Reduce unsafe from STJ

[EgorBo]

Let's see if we can make System.Text.Json 100% unsafe code free (including what we might mark as "requires unsafe" via dotnet/designs#330), could be a good example for community lib authors who heavily rely on Unsafe when implementing serializers.

There are few things left, but they're doable, except for these two:

1. [module: SkipLocalsInit] requires AllowUnsafeBlocks, but I suspect we never measured perf impact of it, for STJ specifically. It mainly just improves non-constant stackallocs. STJ doesn't have those. It uses constant-sized (256 bytes) ones. 256 bytes should be zeroeable with a few instructions (unrolled) - still not free, but perhaps if it doesn't have an impact on all of our dotnet/performance benchmarks we can remove it?
2. Lack of TextEncoder.FindFirstCharacterToEncode(Span) overload. What is interesting, that there is TextEncoder.FindFirstCharacterToEncodeUtf8(Span<byte>) but no safe one for UTF-16. I'll file an API proposal

I did not touch unsafe for .NETStandard/older TFMs, was focusing on .NET 10.0 STJ.

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[bartonjs]

The previous code was asserting in the drain, and you're not. If a new TypeCode were to be introduced, is the exception that comes out of this cast going to be useful enough?

[EgorBo]

The previous code was asserting in the drain, and you're not. If a new TypeCode were to be introduced, is the exception that comes out of this cast going to be useful enough?

if a new type code is added, the default will throw an InvalidCastException? we can only unbox byte out of a boxed byte-enum.
PS: unfortunately, C# does not support statemens in pattern-match switch

[EgorBo]

@eiriktsarpalis @stephentoub does it look good as is? I've reverted the SourceWriter change

[jkotas]

This method is safe for callers. Move this unsafe into the method so that it wraps the unsafe code only?

[EgorBo]

I ended up duplicating code since netfx needs unsafe context in two places

[EgorBo]

Addressed the feedback, thanks!