feat: Improve Chen diagram management

According to `Chen` diagram and new PlantUML Keywords from:
- #1718

Add `Chen` diagram on:
- Syntax _(improve Language Descriptor (to follow plantuml/backlog#4))_
- Sub-diagram
- Readme

_(Similar to #1669)_
_[FYI @Benjamin-Davies]_

README.md

@@ -60,7 +60,9 @@ PlantUML is a component that allows you to create various UML diagrams through s
   - [MindMap diagram](http://plantuml.com/mindmap-diagram)
   - [WBS (Work Breakdown Structure)](http://plantuml.com/wbs-diagram)
   - [Mathematical Notations (AsciiMath, JLaTeXMath)](http://plantuml.com/ascii-math)
-  - [IE/ER (Information Engineering/Entity Relationship)](http://plantuml.com/ie-diagram)
+  - Entity Relationship (ER) diagram
+    - [Information Engineering (IE) diagram](http://plantuml.com/ie-diagram)
+    - [Entity Relationship (ER) diagram (Chen's notation)](http://alphadoc.plantuml.com/doc/markdown/en/er-diagram)
 
 ### 📣 Additional Features
 

src/net/sourceforge/plantuml/EmbeddedDiagram.java

@@ -122,6 +122,9 @@ public class EmbeddedDiagram extends AbstractTextBlock implements Line, Atom {
 		if (s.equals(EMBEDDED_START + "chronology"))
 			return "chronology";
 
+		if (s.equals(EMBEDDED_START + "chen"))
+			return "chen";
+
 		return null;
 	}
 

src/net/sourceforge/plantuml/syntax/LanguageDescriptor.java

@@ -95,6 +95,7 @@ public class LanguageDescriptor {
 		type.add("json");
 		type.add("action");
 		type.add("process");
+		type.add("relationship");
 
 		keyword.add("@startwire");
 		keyword.add("@startbpm");
@@ -124,6 +125,7 @@ public class LanguageDescriptor {
 		keyword.add("@startregex");
 		keyword.add("@startfiles");
 		keyword.add("@startchronology");
+		keyword.add("@startchen");
 		keyword.add("@endwire");
 		keyword.add("@endbpm");
 		keyword.add("@enduml");
@@ -152,6 +154,7 @@ public class LanguageDescriptor {
 		keyword.add("@endregex");
 		keyword.add("@endfiles");
 		keyword.add("@endchronology");
+		keyword.add("@endchen");
 		keyword.add("as");
 		keyword.add("also");
 		keyword.add("autonumber");