Fix prepack step to unblock release

[jelhan]

ember ts:precompile failed with the following error:

error TS2688: Cannot find type definition file for 'ember__test-helpers'.
  The file is in the program because:
    Entry point for implicit type library 'ember__test-helpers'


Found 1 error.

This is caused by @types/ember__test-helpers version 2.9.3 being a stub without including any actual types. Users should use the types provided by @ember/test-helpers. But those types are not provided by the old version used in this project.

ember ts:precompile is used as prepack step. This blocks releasing a new version.