Restructure code: split features into separate files

aux/placeholder.typ

@@ -0,0 +1,16 @@
+#let panicOnPlaceholder = state("panicOnPlaceholder", true)
+
+// Return the value with a colorful background so it is visibly a placeholder.
+// Has the ability to panic if panicOnPlaceholder is set to true.
+#let placeholder(value) = {
+  highlight(
+    fill: rgb(0xff, 0xa2, 0x9c, 0xff),
+    value
+  )
+
+  context(
+    if panicOnPlaceholder.get() {
+      panic("Found placeholder and panicOnPlaceholder is set.")
+    }
+  )
+}