Add Roslyn source generator

[Code-Grump]

🤔 What's changed?

Adds new Roslyn-based source generator to convert feature files into executable tests.

⚡️ What's your motivation?

🏷️ What kind of change is this?

• :zap: New feature (non-breaking change which adds new behaviour)

♻️ Anything particular you want feedback on?

• The code structures for the modelling of features and scenarios
• The models for test fixtures and methods
• The mechanisms for rendering source code
• The tests to validate surface-level code generation

📋 Checklist:

• [x] I've changed the behaviour of the code
  • [x] I have added/updated tests to cover my changes.
• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.
• [ ] Users should know about my change
  • [ ] I have added an entry to the "[vNext]" section of the CHANGELOG, linking to this pull request & included my GitHub handle to the release contributors list.

Tracking review notes:

• [x] Review note 1: opt-in behavior
• [ ] Review note 2: Delete *.feature.cs files
• [ ] Review note 3: how to configure / editorconfig
• [x] Review note 4: test execution does not utilize incremental gen, always builds (accepted limitation)
• [x] Review note 5: performance degradation of incremental command line build (accepted limitation)
• [x] Review note 6: Namespace of generated classes in feature files in sub folders
• [ ] Review note 7: rename project and nuget package
• [ ] Review note 8: nullable warnings
• [x] Review note 9: <PrivateAssets> and <IncludeAssets>
• [x] Review note 10: breakpoint highlight column alignment issue
• [ ] Review note 11: navigate to feature file from test explorer
• [x] Review note 12: Debugger.Launch()
• [x] Review note 13: scenario changes are not immediately reflected in the generated code
• [x] Review note 14: test discovery does not utilize incremental gen (accepted limitation)
• [x] Review note 15: wrong source info for errors generated for syntax errors
• [x] Review note 16: shifted line/column for errors generated for syntax errors during build
• [x] Review note 17: duplicated errors after build (accepted limitation)
• [ ] Review note 18: missing new lines in generated code of scenario outline
• [ ] Review note 19: Compilation error when scenario outlines used for MsTest v3
• [ ] Review note 20: DataTables and DocString parameters are not supported.
• [ ] Review note 21: The scenario description is not passed to the ScenarioInfo.
• [ ] Review note 22: Disable MsBuild gen variables (e.g. ReqnrollFeatureFiles) when Roslyn gen is active
• [ ] Review note 23: CSharpSyntax.CreateIdentifier special handling
• [ ] Review note 24: Scenario outline parameter substitutions for special cases
• [ ] Review note 25: Scenario outline special example headers
• [ ] Review note 26: Scenario outline empty example headers
• [ ] Review note 27: Scenario outline duplicate example headers
• [ ] Review note 28: Support allowRowTests=false
• [ ] Review note 29: support generator/addNonParallelizableMarkerForTags
• [x] Review note 30: Support backgrounds
• [ ] Review note 31: shared extended gherkin parser / additional validations
• [ ] Review note 32: emitIgnoredExamples config

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