Modernize cmake

[mattangus]

This PR moves to a more modern target based CMake usage. This avoids setting properties globally for potential consumers of this project.

[mattangus]

Additionally, we planned to revisit default compile flags for v0.4.0 release at #48 (e.g., -DNDEBUG, -O3 instead of -O2, etc.), but this will be done at the end after it is feature complete (there are some API changes + new functionalities are still in testing, especially curve offsets and polyline fitting)

To respond to your other comment. CMake already has some optimization levels by default. E.g. running a clean build:

$ cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
...
$ cmake --build build
...
[ 25%] Building CXX object CMakeFiles/bezier.dir/src/utils.cpp.o
/usr/bin/c++  -I/path/to/Bezier/include -isystem /usr/include/eigen3 -O3 -DNDEBUG -std=c++17 -fPIC -MD -MT CMakeFiles/bezier.dir/src/utils.cpp.o -MF CMakeFiles/bezier.dir/src/utils.cpp.o.d -o CMakeFiles/bezier.dir/src/utils.cpp.o -c /path/to/Bezier/src/utils.cpp
..

Note the -O3. Similarly, setting -DCMAKE_BUILD_TYPE=Debug the -g flag will be used by default. I'm happy to include the custom optimization level in my PR. But I would set them via CMAKE_CXX_FLAGS_DEBUG and CMAKE_CXX_FLAGS_RELEASE.