chore(core): split out validators 🙀

feat(developer): add unit tests for meta compiler 🙀

- copy over asserts from 'old' code

#7136

yes, a kmxplus file does not start with a 'sect' !

#7136

#7136

chore: merge master into feature-ldml 🙀

chore(developer): Merge branch 'chore/merge-master-into-feature-ldml' into feat/developer/kmldmlc-meta-tests-and-cleanup

Merge pull request #7153 from keymanapp/refactor/developer/kmldmlc-pure-functional-section-compilers

refactor(developer): each section compiler is a pure function 🙀

feat(core): set -DDEBUG for debug 🙀

auto: increment master version to 16.0.55

- set -DDEBUG for 'debug' meson builds
- default KXPLUS_DEBUG  to 1 for -DDEBUG

#7136

First, a little bit of refactoring to make it cleaner to do unit test
for each section.

Then add some global error values.

Throw in a helper function or two.

Dust off the edges of the CompilerCallbacks object.

Duct tape together validation of the normalization attribute.

And ... eventually we get a nice clean-like unit test for the meta
compiler, with three pretty little fixtures and as-neat-as-you-like-it
assertions.

chore(core): emcc off path for linux

Merge pull request #7151 from keymanapp/refactor/developer/kmldmlc-compiler-separate-files-for-sections

refactor(developer): separate files for kmldmlc sections 🙀

chore: merge master into feature-ldml 🙀

Well, almost. There are still callback side-effects for messages. These
may in future be returned as an array as well, eliminating the need for
a message callback.

But restructures each section compiler so that it is self-contained and
responsible only for writing its own section. Also tightens up the types
for the sections constants.

Last, but not least, moves responsibility for writing the output .kmx
out of compiler.ts and into its callers -- removing one dependency from
the compiler. Reading input files still needs to be done in the
compiler, as there may be multiple input files with `<import>` that we
cannot know about until we start parsing xml.

fix(linux): Allow downgrades for installing build deps

fix(core): ldml: fix validator 🙀

chore(linux): Update debian changelog 🍒

Co-authored-by: Marc Durdin <marc@durdin.net>

- allow keys section to exactly fit in data size, as in #7140

feat(core): ldml: improve file reading/validation 🙀

#7043

Co-authored-by: Marc Durdin <marc@durdin.net>

@keymanapp-test-bot skip

feat(developer): unit test framework for kmldmlc 🙀

feat(core): hextobin tool 🙀

feat(core): use kmldmlc to build test keyboards 🙀

chore: update core/ label

Merge pull request #7000 from keymanapp/chore/developer/keymanweb-compiler-remove-unused-dependencies

chore(developer): remove unused dependencies from KeymanWeb compiler 🛩

chore(core): rename json.hpp to jsonpp.hpp 🛩

auto: increment master version to 16.0.54

feat(linux): Dockerfile for linux builder