Add "def" syntax sugar for named function definitions.

This is technically redundant but it makes functional/procedural code
look a *lot* more natural.

test/language/module_variable/undefined.wren

@@ -1,4 +1,4 @@
-var fn = Fn.new {
+def fn() {
   System.print(a)
   System.print(b)
 }

test/language/module_variable/use_in_function_before_definition.wren

@@ -1,4 +1,4 @@
-var f = Fn.new {
+def f() {
   System.print(variable)
 }