Add Sass cache to gitignore

[steadygaze]

I noticed the .sass-cache directory was appearing when I ran git status after building the template. However, this is a build artifact and should be ignored by git. Also see:

• https://github.com/github/gitignore/blob/main/Sass.gitignore
• https://stackoverflow.com/questions/14934800/why-does-sass-cache-folder-get-created

I figured this is probably happening in the other templates too, so I'll create PRs for them too.

[gbj]

Thanks!