Config file for CMake environment variables

CMake uses a number of environment variables for configuration that a user might want to set for all of their projects.

Examples include CMAKE_GENERATOR and CMAKE_EXPORT_COMPILE_COMMANDS.

Because these are effectively CMake configuration options, it would be nice to have a CMake-specific location to define these variables rather than setting them in the shell. Because !9615 (merged) establishes a user config directory, we could allow a ~/.config/cmake/Environment.json file to provide values for these options and set the environment variables for the process.

A warning could be displayed if the definitions in this file shadow existing environment variables.