Implement VK_LUNARG_direct_driver_loading

Implement VK_LUNARG_direct_driver_loading

VK_LUNARG_direct_driver_loading is an instance extension which allows
applictations to include drivers directly to the loader, instead of relying
on system installed drivers or environment variables specifying where the
desired driver is located. This allows applications to 'ship' a driver with
themselves and easily provide it to the system loader.

Changes related to direct driver loading:
* Source implementation
* Documentation of new extension, interactions with existing driver discovery
mechanism, and issues with implicit layers
* Extensive tests covering many feature combinations.

Changes also made in this commit:
* Add EnvVarWrapper which more cleanly handles setting, modifying, and
removing Environment Variables. This has the effect of allowing all tests to
run from the test executable without leaking env-vars, which previously
caused spurious test failure. Note - CTest runs each test in a separate
process which hides this issue.
* Adds -Wshadow=local for the gcc compiler (only for gcc 7 and up).
The MSVC compiler already enables this check with W4 so it was added for consistency.
* Add <!-- omit from toc --> to Documenation pages. This prevents the
"Markdown all in one" extension from adding labels to the table of contents.
* Add ManifestDiscoveryType::null_dir, to separate tests that want to write
a manifest file to a place the loader normally does not search and tests which
do not write any manifest file at all.
* Updated copyright to 2023 for changed files