feat(glossy): adds hide/hidden modifier to suppress term display

There are cases where you might want a term referenced (ie from the
Glossary/Index), but don't want to display it. New modifiers
`hide`||`hidden` do exactly that.

glossy/tests/hidden-terms/test.typ

@@ -0,0 +1,33 @@
+#import "/lib.typ": *
+
+#let myGlossary = (
+  html: (
+    short: "HTML",
+    long: "Hypertext Markup Language",
+    description: "A standard language for creating web pages",
+    group: "Web"),
+  css: (
+    short: "CSS",
+    long: "Cascading Style Sheets",
+    description: "A language used for describing the presentation of a document",
+    group: "Web"),
+  tps: (
+    short: "TPS",
+    long: "test procedure specification",
+    description: "A document on how to run all the test procedures"),
+   unused: "Unused term, which shouldn't print in the glossary."
+)
+
+#show: init-glossary.with(myGlossary)
+
+#set page(height: auto, width: 6.5in, margin: 1em, numbering: "1")
+
+= Hiding terms
+
+Here's a hidden term: @html:hide
+
+Here's another hidden term: @css:hidden
+
+And here's a shown term: @tps
+
+#glossary()