feat(core): hextobin tool

The hextobin tool is used to help us construct invalid .kmx files
without having to either hack these into the compiler, or write them
with a hex editor. It feels a little bit of overkill, but this makes it
easy to (a) work with our intended file structures, and (b) review any
changes to these invalid keyboard tests.

Note: I did do a cursory search for a hex2bin or hextobin that we could
easily use but did not find a good simple cross-platform tool that we
could consume (`xxd -r` would work but AFAICT constrains our format to a
dump which is much harder to work with and read -- I wanted to be able
to annotate with file structures.)

For an example, see ik_000_null_invalid.txt.

This means that building the invalid keyboards for tests requires node,
but that should be fine given we need node to build the valid keyboards
anyway.

I've put hextobin under core/tools but it could just as well live under
common/tools in the future, if we find it useful for other project unit
tests.