add examples of complex types and type constraints

variables/complex_types/README.md

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+# Complex Types
+
+A complex type is a type that groups multiple values into a single value. Complex types are represented by type constructors, but several of them also have shorthand keyword versions.
+
+There are two categories of complex types: collection types (for grouping similar values), and structural types (for grouping potentially dissimilar values).
+
+https://www.terraform.io/docs/language/expressions/type-constraints.html#complex-types

variables/complex_types/main.tf

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+variable "structural-type" {
+  type = object({
+    object = object({
+      string = string
+      number = number
+      bool   = bool
+    }),
+    list = list(any),
+    map  = map(string),
+    set  = set(number)
+  })
+}
+
+output "structural-type" {
+  value = var.structural-type
+}