Add optional regex constraints

examples/regex/README.md

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+# Regex example
+
+Regex constraints are optional. Run this example with the `regex` feature enabled:
+
+```sh
+cargo run -q -p decodal --features regex -- examples/regex/main.dcdl
+```
+
+Without the feature, regex validation returns an unsupported feature diagnostic.

examples/regex/main.dcdl

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+let
+  Identifier = String & /^[A-Za-z_][A-Za-z0-9_]*$/;
+  ApiName = Identifier & /^api_/;
+in
+{
+  service = ApiName default "api_gateway";
+}