Update 'Enum Type Definitions'

Enum-Type-Definitions.md

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+# Syntax
+
+```
+enum ro Weekday:Int
+	sunday
+	monday
+	tuesday
+	wednesday
+	thursday
+	friday
+
+enum ro NamedColor:U32
+	red   0xFF0000
+	green 0x00FF00
+	blue  0x0000FF
+```
+
+Each line under an enum represents a member and is composed of a name, and an optional initialization value.
+
+Enum members are accessed similarly to object members:
+
+```
+set day:Weekday Weekday.thursday
+```
+
+# Semantics
+Enums "enumerate" a pre-existing type, most likely an Int.
+
+Enum members can have explicitly defined values, but if they don't, their value
+is just incremented up from zero. The order in which members are listed in matters.