Remove the `<12.0.0` restriction re: Node.js version in the `"engines"`
settings for all the packages in the monorepo that had that restriction.
Add missing `"engines"` settings in `packages/plugins/snark/package.json`.
Adjust the Azure Pipelines config to include builds for Node.js v12.x and
v13.x.
Bump `solc` to `0.4.26` in `dapps/tests/app` and `dapps/tests/contracts`. It
was discovered that older versions suffered a fatal `Maximum call stack size
exceeded` error when run on Windows with Node.js v12.x or newer. Display a
warning re: the bad combo (solc version + Windows + Node version) if it's
detected at runtime.
Adjust the root `yarn.lock` so that the `sha3` transitive dependency resolves
to a newer version that is compatible with Node v13.x.