feat(developer): verify keyboard versions in kmc-package

Refactor the `extractKeyboardVersionFromKmx` function into a new class
and add extra version checking. This caused a bit of a cascade of test
failures due to some of the 'invalid' fixtures actually being invalid
in multiple ways, so the fixtures have been corrected to only be wrong
in a single way -- the way they are supposed to be broken. This means
additional fixture files in the 'invalid' folder.

Now updates keyboard version metadata for all keyboards in the package.

Upgrades `WARN_FollowKeyboardVersionButNoKeyboards` to
`ERROR_FollowKeyboardVersionButNoKeyboards`, as this leads to invalid
package metadata on build.

Renames `ERROR_KeyboardFileNotFound` to
`ERROR_KeyboardContentFileNotFound` to better reflect that no `<File>`
in the package is found to match a given `<Keyboard>` entry.

Adds `ERROR_KeyboardFileNotFound` when a referenced .kmx does not exist.

Adds and updates corresponding unit tests.