Commit conventions are not enforced

[rowanmanning]

If you're developing locally then commit conventions are enforced through a husky commit-message hook, however this doesn't catch when someone commits via the GitHub UI. I think we need to add a step to the build which fails it if any of the commits in a PR don't match our conventions, otherwise Release Please won't be able to calculate a correct release version.

Node.js version: All
Impacted package(s): All

Steps to reproduce

1. See this PR's build passing despite it not using conventional commits

Expected behaviour

I expect the build to fail, ideally with a useful message which points to the relevant part of the contributing guide.