[.NET] Adopt c# 10 file scoped namespaces

[Romfos]

🤔 What's changed?

Adopt c# 10 file scoped namespaces

documentation: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-10.0/file-scoped-namespaces

⚡️ What's your motivation?

Adopt modern c# language feature to simplify code base

🏷️ What kind of change is this?

• 🏦 Refactoring/debt/DX (improvement to code design, tooling, etc. without changing behaviour)

♻️ Anything particular you want feedback on?

no changes in product. code style changes only

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• I've changed the behaviour of the code
  • 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 "Unreleased" section of the CHANGELOG, linking to this pull request.

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This text was originally generated from a template, then edited by hand. You can modify the template here.

[Romfos]

Parser.cs is not changed because it has this header

// ------------------------------------------------------------------------------
//  <auto-generated>
//      This code was generated by Berp (http://https://github.com/gasparnagy/berp/).
// 
//      Changes to this file may cause incorrect behavior and will be lost if
//      the code is regenerated.
//  </auto-generated>
// ------------------------------------------------------------------------------

[mpkorstanje]

You can edit dotnet/gherkin-csharp.razor to apply the name spaces to the generated code. Instruction on how to generate the parser can be found in CONTRIBUTING.md.

[clrudolphi]

Other than the 1 spacing/formatting item found (commented elsewhere), this looks good.
It would be nice, for consistency's sake, to complete this PR by adding in the changes required of the razor file that @mpkorstanje suggested. I can walk you through them if you need it.

[Romfos]

please squash this PR during merging

[Romfos]

I propose to make changes in code-generation later if needed, because I'm planning to propose another changes in this library, including code style changes