feat: Add CancellationToken support for RunBuild.Exec

[filzrev]

This PR add CancellationToken support toRunBuild.Exec method.
It's expected to be used by docfx watch command.

What's Changed in this PR

• Add CancellationToken parameter to methods and use token on operations that take times.

  • for/foreach operations
  • Parallel.ForEach operations
  • AsParallel() operations

• DocumentBuildContext.cs

  • Add CancellationToken property.
  • Change some constructor's access modifiers from public to internal/private that used by tests.

• DocumentExceptionExtensions.cs

  • Add logics to handle AggregateException on cancellation.

Breaking Changes

This PR change some public method signatures.
The user who using Docfx.Plugins package need to update versions.

Test
Manually run cancellation tests with various delay patterns.
And confirmed operation is canceled with OperationCanceledException based exception (including TaskCanceledException)

[yufeih]

Thank you @filzrev !