Return Task from TextWriter.WriteAsync/WriteLineAsync(ReadOnlyMemory<char>)

[SimonCropp]

Bug

The polyfills for TextWriter.WriteAsync(ReadOnlyMemory<char>, CancellationToken) and TextWriter.WriteLineAsync(ReadOnlyMemory<char>, CancellationToken) returned ValueTask. The BCL methods return Task (checked in the netcoreapp2.1, netstandard2.1 and net11 reference assemblies). Code such as Task t = writer.WriteAsync(memory);, Task.WhenAll(writer.WriteAsync(memory), ...) or .ContinueWith(...) compiled against the BCL but not against the polyfill, and because of the return type the polyfills were only available when System.Threading.Tasks.Extensions was referenced.

Fix

• Both polyfills return Task.
• They no longer sit behind FeatureValueTask; FeatureMemory is enough.
• A cancelled token now produces a cancelled task (Task.FromCanceled) instead of throwing synchronously, which is what the BCL does and what the neighbouring WriteAsync(string, CancellationToken) polyfill already does.

Coverage

• TextWriterWriteMemoryAsyncReturnsTask assigns the results to Task (does not compile against the old signatures).
• TextWriterWriteMemoryAsyncCancelled asserts a pre-cancelled token yields a cancelled task and writes nothing.
• Consume.cs assigns both overloads to Task and no longer requires FeatureValueTask for them.
• Tests run on net48 (polyfill) and net10.0 (BCL); Consume built for net461, netstandard2.0, netcoreapp2.0, netcoreapp2.1 and net10.0.
• Split files and api_list.include.md regenerated.

Note: this is a source-breaking change for callers that stored the result in a ValueTask.