feat(fastzip): allow custom filesystemscanner

[piksel]

Fixes #676

Or rather, allows it to be handled with a derived class:

  new FastZip().CreateZip(outStream, srcDir, recurse: true, new CustomFileSystemScanner(filter), leaveOpen: true);

class CustomFileSystemScanner:  FileSystemScanner 
{
	static IEnumerable<string> FixupNet5Enumeration(IEnumerable<string> items)
		=> items
			.Select(t => t.TrimEnd('\0'))
			.Where(t => {
				var itemName = Path.GetFileName(t);
				return itemName != "." && itemName != "..";
			});

	protected override IEnumerable<string> GetDirectories(string directory) 
		=> FixupNet5Enumeration(base.GetDirectories(directory));

	protected override IEnumerable<string> GetFiles(string directory) 
		=> FixupNet5Enumeration(base.GetFiles(directory));

	public CustomFileSystemScanner(string ff): base(ff) {}
	public CustomFileSystemScanner(string ff, string df): base(ff, df) {}
	public CustomFileSystemScanner(IScanFilter ff): base(ff){}
	public CustomFileSystemScanner(IScanFilter ff, IScanFilter df) : base(ff, df) {}
}

I certify that I own, and have sufficient rights to contribute, all source code and related material intended to be compiled or integrated with the source code for the SharpZipLib open source product (the "Contribution"). My Contribution is licensed under the MIT License.

[codecov]

Codecov Report

❌ Patch coverage is 54.05405% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.90%. Comparing base (612969e) to head (14cdc85).
⚠️ Report is 45 commits behind head on master.

Files with missing lines	Patch %	Lines
.../ICSharpCode.SharpZipLib/Core/FileSystemScanner.cs	44.44%	30 Missing ⚠️
src/ICSharpCode.SharpZipLib/Zip/FastZip.cs	80.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
+ Coverage   73.79%   73.90%   +0.10%     
==========================================
  Files          68       68              
  Lines        8332     8301      -31     
==========================================
- Hits         6149     6135      -14     
+ Misses       2183     2166      -17     

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[piksel]

Needs more tests, but otherwise looks OK.

[piksel]

Check direcotryFilter_ for null. Perhaps by

Suggested change

	var subDirectories = GetDirectories(directory)
	.Where(d => d != null && directoryFilter_.IsMatch(d));
	Func<bool, string> filter = directoryFilter_ == null
	? (d) => true
	: (d) => directoryFilter_.IsMatch(d);
	var subDirectories = GetDirectories(directory)
	.Where(d => d != null && filter(d));