Zephyr now has some modules that get placed under /optional/modules
instead of /modules. Added that to .gitignore so Git doesn't try to
treat those like Git modules.
VS Code can't share two folders with a Docker container at the same time, so
to build with -DZMK_CONFIG the zmk-config repo needs to be a subfolder of the
zmk repo. Added zmk-config to .gitignore to allow this.