Spike: Switch from lerna to pnpm workspaces

Our current lerna-based setup is very inefficient. Most of the pull requests in this repository are created by RenovateBot to update package-lock files, our git history is full of RenovateBot commits that make it difficult to spot relevant changes.

Let's investigate how to switch from lerna to [pnpm](https://pnpm.js.org/) and use a single root package-lock file for the entire monorepo.

References:
- pnpm workspaces: https://pnpm.js.org/en/workspaces
- Travis CI setup: https://pnpm.js.org/en/continuous-integration#travis

Topics to investigate:
- How to release new versions using conventional release process? Can we perhaps use https://github.com/atlassian/changesets? Related: https://github.com/pnpm/pnpm/issues/2225
- Does RenovateBot support pnpm? If not, is there any other bot we could use instead? E.g. GitHub's [Dependabot](https://github.com/dependabot). Related: https://github.com/renovatebot/renovate/issues/2015

Start with the pnpm workspaces and Travis CI references in the issue, then examine the current Lerna, package-lock, RenovateBot, and release workflows. Done means documenting a viable migration plan, including release handling and dependency-update support, rather than making an implementation change.

Written by the indexing model from the issue text.