Add string syntax attribute to `.AppendFormat()`

[Atulin]

Using [StringSyntax] attribute like so:

public void AppendFormat<T1>([StringSyntax(StringSyntaxAttribute.CompositeFormat)]string format, T1 arg1)

would make it so that IDEs can provide better syntax highlighting:

Docs: https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.codeanalysis.stringsyntaxattribute?view=net-10.0