refactor: Move Meson build targets to top level directory

[WillAyd]

This makes the Meson build generate targets more closely to the CMake build. Instead of building targets in src/nanoarrow this makes it so it builds things in the build directory root

The only exception is the "nanoarrow_config.h" file which gets built to <build_directory>/src. The CMake installation creates a separate generated directory, but Meson practically limits the structure of your build folder to how the meson.build files are laid out in source. We could create a generated directory in the source if we so cared, or alternately could try to change the include to #include <nanoarrow/nanoarrow_config.> for consistency with all other includes, placing that generated target in the same directory as the rest of the headers

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.99%. Comparing base (41e9e71) to head (ee5df26).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #530      +/-   ##
==========================================
+ Coverage   88.92%   88.99%   +0.06%     
==========================================
  Files          89       89              
  Lines       16339    16290      -49     
==========================================
- Hits        14530    14497      -33     
+ Misses       1809     1793      -16     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[paleolimbot]

alternately could try to change the include to #include <nanoarrow/nanoarrow_config.h>

That will be the case after #508 !

[paleolimbot]

Thank you!