Investigate coverxygen for Cesium Native

[azrogers]

coverxygen is a tool that generates a documentation code coverage report from Doxygen's XML output. This coverage report can be fed to lcov or output in a summary by the tool itself. An example output from this tool, run on Cesium Native, looks like this:

Classes    :  85.1% (223/262)
Defines    :   0.0% (0/34)
Enum Values:  67.1% (98/146)
Enums      :  79.3% (23/29)
Files      :   0.0% (0/419)
Friends    :   0.0% (0/32)
Functions  :  71.8% (1828/2547)
Namespaces :  78.3% (18/23)
Pages      : 100.0% (6/6)
Structs    :  68.9% (279/405)
Typedefs   :  37.1% (33/89)
Variables  :  47.8% (854/1788)
-----------------------------------
Total      :  58.2% (3362/5780)

It would be useful to have this tool around for Cesium Native, if not in CI then at least as a section in the developer's guide README.