chore(developer): improve kmc validation and corresponding tests 🙀

feat(developer): add kmx metadata to kmxplus compiles 🙀

- split validation out into a separate kmx_file_validator.hpp
- kmx_file.h should remain C-only usable

Fixes: #7220

* Add a bunch of tests for keys
* Add tests for version. (improves meta test coverage to 100%)
* A bit more consistency cleanup also of error message functions.
* Adds a compiler failure (FATAL) if compile fails after validate
  passes.
* Overall test coverage now 93.5% lines. Biggest gap still kmx-builder.
  All compiler units are >90%.

Fixes #7273.

Adds 4 kmx system stores to allow existing configuration tools to load
and process KMXPlus files without modification:

* `TSS_NAME` (first `<name>`)
* `TSS_COMPILEDVERSION` (only if `options.addCompilerVersion == true`)
* `TSS_KEYBOARDVERSION` (from `<version number>`)
* `TSS_TARGETS` (currently always 'desktop', will later include more
  targets)

In order for `TSS_COMPILEDVERSION` to be handled correctly, needed to
plumb in the new `CompilerOptions` interface. This could be a separate
PR, but the changes are not huge, so opted to append the changes here.

Updates related tests and fixtures. Refactored some of the e2e helpers
into helpers/index.ts for use with the metadata-compiler tests.

- move VerifyKeyboard into COMP_KEYBOARD::VerifyKeyboard
- change call sites in kmx_file.h

For: #7220

chore(core): verify StartGroup in kmx load 🙀

chore(core): use builder_ for build.sh 🙀

chore: make checksum optional 🙀

chore: merge master into feature-ldml 🙀

chore(web): disable browserstack on non-web-specific builds

chore:  try disabling concurrency for browserstack tests

Merge pull request #7219 from keymanapp/fix/developer/7216-suppress-repeated-warnings-unreachable-code

fix(developer): suppress repeated warnings about unreachable code

fix(developer): compiler mismatch on currentLine

docs(windows): add steps for using testhost debugging