chore(common): add missing deps and files for npm publish

The build/ folder for most packages was missing due to being listed in
.gitignore.

Also, @keymanapp/ldml-keyboard-constants package was not being published
despite being a dependency of @keymanapp/kmc-keyboard.

Instead of relying on npm's use of .gitignore / .npmignore (which has
seriously wonky behaviours), we list the files that should be included
in package.json.

NPM Wonky behaviours: it seems that .npmignore causes .gitignore in the
same folder to be ignored. But, higher level .gitignore files still
affect the files included in the package, which makes
specification-by-exclusion painful (negating exclusions, etc). We do not
recommend use of .npmignore anywhere for this reason.