remove static uses of PrettyMarkupFormatter

[SimonCropp]

PrettyMarkupFormatter is not thread safe

VerifyTests/Verify.Bunit#60

[egil]

Thanks Simon. The particular use in bUnit is actually thread-safe, as the Attribute writing method does not rely on any state, but I am fine with instantiating the class here and add the obsolete flag, so other users dont assume the static instance is thread safe.

@linkdotnet any objections to merging this?

[SimonCropp]

@egil

The particular use in bUnit is actually thread-safe,

r u certain? is it not possible to call ToMarkupElementOnly from multiple threads?

[egil]

@egil

The particular use in bUnit is actually thread-safe,

r u certain? is it not possible to call ToMarkupElementOnly from multiple threads?

As far as I can tell, ConvertToString(attribute) calls base.Attribute(), and that method has no references to fields in the base class. Thus, I think it is safe.