Add @hyde-ignore directive

We need a command that can be used in-source that tells hyde to ignore whatever AST node the clang driver has found. This would likely be recursive (e.g., ignore a class and the member fields and variables would be ignored, too.)

Start by tracing how hyde receives AST nodes from the clang driver and how in-source directives are currently represented, if supported. Define the recursive behavior for ignored nodes, then verify that ignored nodes and their members or variables are excluded from generated documentation.

Written by the indexing model from the issue text.