Remove Lerna config and lift MST sub-package up

[coolsoftwaretyler]

This is the last step to making MST no longer a monorepo.

With #2091, we finally removed the last of the sub-packages out of this repository. But we are currently still set up in Lerna, and with sub-packages. We won't change that until we close most (or hopefully all) of the open PRs in the repository to avoid gnarly merge conflicts.

This issue should get closed once we have a PR to actually remove all the monorepo structure. We put out a discussion post over here: #2089 to let folks know to anticipate it.