support .gocovignore files

To allow each module to define what files or folder to ignore, a .gocovignore would be useful.

For example, I want to ignore anything under hack/* folder because that's where I just put some tools.
so I should be able to add a .gocovignore file at the root of my project, with `hack/` in it.

syntax could be like .gitignore, or simpler if needed.

Start by reviewing how gocover currently discovers files and folders to measure. Define the expected .gocovignore location and whether its syntax follows the example hack/ pattern or .gitignore rules. Done means ignored paths are excluded from coverage while other project files continue to be measured, with behavior verified by tests.

Written by the indexing model from the issue text.