Configure Semantic Release

If we use [Semantic Release](https://github.com/semantic-release/semantic-release) to release this repository, we'll get automatically-incremented releases, version tags, and changelogs on each merge to `master`, at the low cost of having to follow a commit prefix convention. This takes minimal configuration:

* a semantic release config (in `package.json`)
* a couple of lines in the release CI task to dispatch SemRel and possibly access to the credentials it needs to commit tags back to the repo
* [optional] a dry-run task in PR pipelines to show committers what to expect on merge

We also have the option, which I'd recommend, of using their `rc` branch pattern:

* all PRs target the `rc` branch, not `master`.
* On merge to `rc`, PRs release a new `rc` version for the next applicable version. For example, if we were to start following `v2.0.0`, a PR whose commits trigger a minor release would be tagged `v2.1.0rc0`.
* Full versions are released simply by merging `rc` into `master` - no other chores needed. Then, `v2.1.0rc0` would become `v2.1.0`.