build: migrate to monorepo (for vue-jest@4)

[nogic1008]

⚠️ DO NOT merge this PR with "Squash & Merge" method!! ⚠️ It breaks package.json histories.

This PR is a part of #355.

Changes

• setup monorepo (use yarn workspace)
• move e2e/__projects__/* projects to e2e/2.x/*
• move package.json and lib/ to packages/vue2-jest/
• change package to vue2-jest@26.0.0
• fix e2e test for monorepo

[lmiller1990]

Wow, great work! I'll merge this on up.

We will do a release soon to test this all out and make sure we didn't miss anything, but seems fine!