Add missing peer dependency @babel/core@^7.0.0-0

[bantic]

When installing using yarn, there are many console warnings about an unmet peer dependency on @babel/core@^7.0.0-0 for some of the deps, such as @babel/plugin-proposal-object-rest-spread.
These warnings also show up when consumers do their yarn install, e.g.:

warning "ember-table > @babel/plugin-proposal-nullish-coalescing-operator@7.14.5" has unmet peer dependency "@babel/core@^7.0.0-0".

This PR adds an explicit dependency on babel/core to silence those warnings.

Doing a yarn install in ember-table locally before:

And after:

I am taking inspiration from yarnpkg/yarn#5347 (comment) that recommends that package maintainers add the missing peer dep locally, when the yarn warning originates from their package.

[bantic]

Test failures appear to be due to changes in Ember v4 (which is "beta" at the time CI ran).

[mixonic]

I've opened #911 to pin the CI at 3.28, and force pushed a rebase of this branch.