With help from @linonetwo, we now have Vercel set up to make a preview deployment of every commit to the TiddlyWiki5 repository. This should make it much easier for contributors to check that their PRs are working as expected without having to set up a TW5 build environment on ones own machine.
For example, here are the deployment details for the PR for parameterised transclusion:
https://vercel.com/jermolene/tiddlywiki5/3v8JsEW16yzzGt4mShZ7NBDeGnn7
The preview deployment is linked from a comment in the PR on GitHub:
Note that preview deployments do not occur retrospectively for existing branches/PRs. If you own a PR you’ll need to make a commit to to trigger the creation of a preview build.