System.Text.Json.JsonProperty.WriteTo uses get_Name, calling
JsonElement.GetPropertyName() which would allocate a string.

Use ReadOnlySpan<byte> from the underlying UTF8 json, when possible,
by adding helper methods into JsonDocument & JsonElement.

Fix dotnet#88767

Current code unescapes & escapes property names and uses ToArray.
Avoid alloc by adding internal GetRaw/WriteRaw methods.

Original code doesn't handle GetRaw/WriteRaw on escaped
property names correctly.

Allocations are further avoided when the property name is shorter than
JsonConstants.StackallocByteThreshold, by inlining
JsonReaderHelper.GetUnescapedSpan.

Shorten names of new methods;
Add a test of writing out special names.