Fixes to C# branch: unit tests and code generation

[rzubek]

Hi! I'm glad to have found this project, this parser generator is awesome!

I wanted to contribute some improvements, in the hope that the C# variant gets finalized and merged into the main branch - it would be amazing to have C# coverage in addition to the other languages.

I see that there was an old PR#70, which got merged into c-sharp branch, but some of the desiderata were still waiting to be addressed. This pull request addresses the following:

• C# generator now creates only one file per grammar, which is compatible with C# standard practice (unlike Java which requires a file per class)
• Upgraded project files (makefile and test csproj) to .NET Core 6/8 for better multi-platform compatibility

Regarding .NET - these changes are only relevant for people running make and unit tests. I upgraded project and test files, because Core 6/8 are the only ones with current long-term support - older versions of .NET are now past their end-of-life date, and running them will be increasingly difficult.

However, this is only for testing. The generated C# code is highly backwards-compatible, and I would expect it to work with .NET going back a decade or more.

Testing:

I ran unit tests on Ubuntu (x86) with dotnet sdks 6.0 and 8.0:

$ make clean test-cs
find examples -name '*.class' -o -name '*.pyc' -exec rm {} \;
find test/grammars -type f -a ! -name '*.peg' -a ! -name __init__.py -exec rm {} \;
./bin/canopy --lang cs test/grammars/choices.peg
./bin/canopy --lang cs test/grammars/extensions.peg
./bin/canopy --lang cs test/grammars/node_actions.peg
./bin/canopy --lang cs test/grammars/predicates.peg
./bin/canopy --lang cs test/grammars/quantifiers.peg
./bin/canopy --lang cs test/grammars/sequences.peg
./bin/canopy --lang cs test/grammars/terminals.peg
cd test/cs && dotnet test --framework net6.0
  Determining projects to restore...
  All projects are up-to-date for restore.
  canopy -> /home/rob/code/canopy/test/cs/bin/Debug/net6.0/canopy.dll
Test run for /home/rob/code/canopy/test/cs/bin/Debug/net6.0/canopy.dll (.NETCoreApp,Version=v6.0)
Microsoft (R) Test Execution Command Line Tool Version 17.8.0 (x64)
Copyright (c) Microsoft Corporation.  All rights reserved.

Starting test execution, please wait...
A total of 1 test files matched the specified pattern.

Passed!  - Failed:     0, Passed:   127, Skipped:     0, Total:   127, Duration: 339 ms - canopy.dll (net6.0)

I hope this is useful in bringing this branch closer to merging with main - and please let me know if I can help with C# support!

[rzubek]

By the way, one thing that's left unresolved is namespaces.

This port inherits the Java namespace definition, which follows the directory structure. This is a requirement in Java, but almost always the incorrect thing to do in C#, where namespaces are expected to follow logical project structure.

Maybe it would make sense to add another command-line argument to the compiler, which would pass it over to the builder, to let the user customize the namespace?