fix: allow nested zmk-config for dev container

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.

.gitignore

@@ -3,5 +3,6 @@
 /modules
 /tools
 /zephyr
+/zmk-config
 /build
 *.DS_Store