fix(developer): keep kmc-kmn messages within namespace

Fixes #8998.

kmc-kmn messages were broken down into sub-namespaces, but the range
overlapped the reserved namespace mask range. Fixed the message values,
added a new test to the verifyCompilerMessagesObject function to verify
that messages don't creep outside their namespace, and defined new
masks (in CompilerErrorSeverity enum... hmm) to help.

I opted to remove the leading `0` in the messages for kmc-kmn as I had
to correct a number of them anyway, but not the messages in remaining
units at this time (it's not an error, but just slightly misleading as
we only have 12 bits, not 16 to play with).

It is safe to reassign these messages as they were only assigned in 17.0
alpha.