Add loader enable/disable env vars

Add loader enable/disable env vars

Add new environment variables that will allow the loader to filter
layers and drivers in specific ways.
This control should give developers ways to more quickly narrow down
issues with a layer or driver.
Also, it should give CI environments a mechanism to selectively enable
only drivers and layers that are needed for testing.

Add tests to support validating the new changes.

Add documentation that describes the new changes and also create new
loader debugging markdown document to help people debug issues with
drivers or layers using the new filter enums.

Rename the old get_environment.* source files to loader_environment.*.
Then put all the environment variable helpers from loader.c into the
new files.