The version of a build is now derived from both the `version` field in
`package.json` and the requested build type and version. The build type
and version are added onto the manifest version as a suffix, according
to the SemVer prerelease format.
We already have support in the extension for versions of this format,
but to apply a Flask or Beta version required manual updates to
`package.json`. Now it can be done just with build arguments.
A `get-version` module was created to make it easier to generate the
version in the various places we do that during the build. It was
created in the `development/lib` directory because it will be used by
other non-build development scripts in a future PR.
The `BuildType` constant was extracted to its own module as well, and
moved to the `development/lib` directory. This was to make it clear
that it's used by various different development scripts, not just the
build.
