Add Actions pipeline to deploy production artifacts to GitHub Pages, fix relative paths for TTF font loading in Webpack

[crocodele]

This PR enables automatic deployment of the static Planet Generator files to GitHub Pages upon merge to master branch, with the URL being https://pistolshrimpgames.github.io/ProceduralPlanet/. The deployed files shouldn't mess with the Pistol Shrimp documentation site at https://pistolshrimpgames.github.io/, as long as no-one adds a directory named ProceduralPlanet to the documentation site. 🙂

Before merging this PR, please go to the repository settings > Pages and switch the source from Deploy from a branch to GitHub Actions.

Dry-run (will delete after merge):

• Planet Generator: https://crocodele.github.io/ProceduralPlanet/
• Dummy documentation site: https://crocodele.github.io/

I included the changes from https://github.com/pistolshrimpgames/ProceduralPlanet/pull/15 in this PR, since both are touching the Webpack build config.