feat(core): use kmldmlc to build test keyboards

Removes .kmx from repo and builds them from the source .xml files using
kmldmlc (building that if necessary also with a poor fella's dependency
check for now), and generally a lot of cleanup of the meson.build files
for the ldml tests.

Moves keyboards to a subfolder of the ldml unit test folder to keep them
neat and tidy and avoid embedding huge files into ldml.js for test of
WASM.

Note: 000_null_keyboard and 002_null_invalid do build successfully, but
the core ldml keyboardprocessor currently fails on them, because they
are missing keys and vkey sections.