Remove cmate in favour of native CMake + CPM

[daantimmer]

🤔 What's changed?

• Removed custom cmate scripts and replaced it with native cmake instead
• Use Cmake Package Manager (CPM) to (optionally) download nlohmann/json
• Update cpp.hpp.erb codegen to indent properly
• Update cpp.hpp.erb to treat all functions equal on where the return type should be located (same line instead of separate line)
• Update codegen to write to separate folders. Separating custom maintained sources/headers and generated files to make the maintenance work more streamlined. (The manually maintained sources are easily overlooked)

⚡️ What's your motivation?

The custom cmate script adds no value and only confusion on maintaining the cpp codebase. Secondly its an unmaintained script by a third party.

As for the codegen, there were some inconsistencies.

🏷️ What kind of change is this?

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

♻️ Anything particular you want feedback on?

I am not too sure about the separation of generated vs non-generated code. It makes reading and maintaining a lot easier. However it does mean that this is a very large change because of all the file moves.

Regarding the checklist's #pragma once todo item. This is a highly debated topic in C++ land. However as a public library we should follow the core guidelines as closely as possible, and thus follow the rule: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#sf8-use-include-guards-for-all-header-files

I can, if preferred, do this in a separate pull request.

On that note I'll also add tooling (in a separate PR) to check for compliancy with the c++ core guidelines.

📋 Checklist:

• I agree to respect and uphold the Cucumber Community Code of Conduct
• 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.
• Update supporting code to maintain a consistent coding style to be done in separate PR
• Update test-cpp to actually use different clang and gcc versions to be done in separate PR
• Replace #pragma once with proper include guards. to be done in separate PR
• Tooling for checking core guidelines

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[daantimmer]

I've verified these changes do not affect cucumber/gherkin -> https://github.com/cucumber/gherkin/pull/633/checks and are fully backwards compatible.

[mpkorstanje]

LGTM, but I don't quite understand the caching setup for Github Actions.