test(glossy): switch to typst-test, add tests

- move examples/readme.typ into a test (demo)
- add test for sorting (both ways)
- add test to generate theme gallery
- add theme gallery image to README

preview/glossy/0.2.1/typst.toml

@@ -8,4 +8,4 @@ description = "A very simple glossary system with easily customizable output."
 categories = ["model"]
 keywords = ["glossary", "index", "dictionary", "gloss"]
 repository = "https://github.com/swaits/typst-collection"
-exclude = ["thumbnail.png", "examples/readme.pdf"]
+exclude = ["thumbnail.png", "themeshots.pjng", "tests"]