Add C/C++ development flake with some sane defaults.

[joelbenway]

This is my stab at a flake for a C/C++ toolchain. Here's my reasoning for adding what I did:

• A commented out override of the stdenv with an instruction on how to change the compiler.
• clang-tools, this package includes the excellent and ubiquitous clangd, clang-tidy, clang-format utilities
• cmake, the most popular build system
• codespell, not specifically a C/C++ tool but I like it
• conan, popular C/C++ package manager
• cppcheck, static analysis tool that pairs well with clang-tidy as they cover different ground
• doxygen, popular tool that generates docs from annotated source code
• gdb, debugger
• gtest, this is google's unit test framework library, I just wanted to show you can add libraries with nix for your build system to find so I chose a popular development time one.
• lcov, test coverage tool
• vcpkg, the other popular package manager
• vcpkg-tool, used with vcpkg

I think I plumbed all of this in correctly and updated the necessary docs. You might notice vcpkg packages are missing versions. The tool returns '2999-12-31-unknownhash' when prompted for a version which is obviously wrong. This one appears relatively new to Nix.

[lucperkins]

@joelbenway One little snag here is that it doesn't look gdb is supported on macOS (as reported by nix flake check --all-systems --no-build). If you can make adding that to the shell conditional on the OS, +1 from me.

[lucperkins]

Just need to make this more robust re: Linux vs. macOS

[lucperkins]

Looks good to me. Could you rebase against main? It'll be good to go once that happens.

[joelbenway]

Looks good to me. Could you rebase against main? It'll be good to go once that happens.

Should be ready to merge