CMake and testability improvements

[phprus]

CMake and testability improvements.
I apologize for the large number of changes in single PR.

Purpose of changes:

1. Support for compiling and running tests if argparse is included via add_subdirectory.
2. Reusing code inside CMakeLists.txt project files.
3. Enable compilation of all components in GitHub Actions CI.

@p-ranav, Please review my PR.

[topazus]

@phprus Maybe it will be good to add and install CMake target file argparseTargets.cmake? I am not familiar about CMake. I see this from other projects, like cxxopts. https://github.com/jarro2783/cxxopts/blob/554396be3b19bff45384a3c6307a1eb9a3497940/cmake/cxxopts.cmake#L118-L129

[threeal]

Isn't just putting /W4 will guarantee it will always replace other lower level warning level?

[threeal]

I don't see any differences in here, why don't we use this instead:

Suggested change

	elseif(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
	set(ARGPARSE_PEDANTIC_COMPILE_FLAGS -Wall -Wno-long-long -pedantic -Wsign-conversion -Wshadow -Wconversion -Werror -Wextra)
	elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
	set(ARGPARSE_PEDANTIC_COMPILE_FLAGS -Wall -Wno-long-long -pedantic -Wsign-conversion -Wshadow -Wconversion -Werror -Wextra)
	elseif(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
	set(ARGPARSE_PEDANTIC_COMPILE_FLAGS -Wall -Wno-long-long -pedantic -Wsign-conversion -Wshadow -Wconversion -Werror -Wextra)

[threeal]

I think it's better to leave it like the previous implementation, no need to add another step for creating a directory:

Suggested change

	working-directory: ${{runner.workspace}}/build
	shell: bash
	run: cmake -DARGPARSE_BUILD_SAMPLES=ON -DCMAKE_BUILD_TYPE=Debug ${{ matrix.cmake_opts }} $GITHUB_WORKSPACE
	run: cmake . -B build -DARGPARSE_BUILD_SAMPLES=ON -DCMAKE_BUILD_TYPE=Debug ${{ matrix.cmake_opts }}

Also setting shell: bash is not needed here.

[threeal]

No need to set the working-directory, we can just use this:

Suggested change

	working-directory: ${{runner.workspace}}/build
	run: cmake --build . --config Debug --verbose
	run: cmake --build build --config Debug --verbose

[threeal]

The same here, we can also use --test-dir instead of setting the working-directory and use --output-on-failure instead of CTEST_OUTPUT_ON_FAILURE:

Suggested change

	working-directory: ${{runner.workspace}}/build
	run: ctest -C Debug -V
	env:
	CTEST_OUTPUT_ON_FAILURE: True
	run: ctest --test-dir build -C Debug -V --output-on-failure