Make it easy to deploy TerriaMap to GH Pages

[meiqimichelle]

We got a few notes from #773 about speedbumps when trying to deploy a TerriaMap to GitHub Pages.

This PR makes two changes to (hopefully) make this process a little more straightforward for future travelers. It:

1. Adds the packageManager field to package.json and pins yarn@1.22.22 so that Corepack can manage it automatically.

2. Adds a GitHub Pages deployment workflow to this repo that uses Corepack (no npm dependency), our existing --baseHref gulp flag, and GitHub's official Pages deployment actions. Triggers on push to main (or manual dispatch).

Forks can enable GitHub Pages in their repo settings and this workflow will handle the rest.

Note: for this to work in this repo, we also need to check our settings. We'd need:

• Settings → Pages → Source -- set to "GitHub Actions" (not "Deploy from a branch")
• Settings → Actions → General -- workflow permissions would need to allow Pages deployment
• [EDIT: ...and also enable GitHub Pages, if it isn't already enabled]

There may be Reasons some or all of this is not a good idea. Feedback welcome.