Web component support (#211)

* Foundational components for reading/processing Wattsi files into classes.

* Introducing tests for compliance with wattsi definitions.

- Several elements have been found to be missing from the code generator.
- A long-term strategy for dealing with obsolete elements will be needed.  Should we remove and change the API?  Or should we deprecate and leave in-place?
- Found one element that was accidentally added to the code generator but never used.  Hopefully.
- Fixes will be applied in following commits.

* Added newer elements/tags and cleaned up invalid Tag class.

- Added DataTag.
- Added SlotTag and TemplateTag to support web components.
- Added global attributes to support web components.
- Removed GenerateTag.  Accidentally introduced by string replacement in code generator.

* Upgrading revapi and maven plugin. Configured revapi to ignore removal of GenerateTag.

- The GenerateTag class was created accidentally.  It does not have any representation in the HTML standard is should not be provided in this library.

code_gen/src/test/java/j2html_codegen/CodeGeneratorComplianceTests.java

@@ -0,0 +1,77 @@
+package j2html_codegen;
+
+import j2html_codegen.generators.TagCreatorCodeGenerator;
+import j2html_codegen.wattsi.ElementDefinition;
+import j2html_codegen.wattsi.WattsiSource;
+import org.jsoup.Jsoup;
+import org.jsoup.nodes.Document;
+import org.junit.Before;
+import org.junit.Ignore;
+import org.junit.Test;
+
+import java.io.IOException;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+import static java.util.Collections.emptyList;
+import static java.util.stream.Collectors.toList;
+import static org.junit.Assert.assertEquals;
+
+public class CodeGeneratorComplianceTests {
+
+    private WattsiSource specification;
+
+    @Before
+    public void setUp() throws IOException {
+        Path source = Paths.get("src","test","resources","2022-01.wattsi");
+        Document doc = Jsoup.parse(source.toFile(), "UTF-8", "https://html.spec.whatwg.org/");
+        specification = new WattsiSource(doc);
+    }
+
+    private Set<String> generatedElements(){
+        Set<String> elements = new HashSet<>();
+        elements.addAll(TagCreatorCodeGenerator.emptyTags());
+        elements.addAll(TagCreatorCodeGenerator.containerTags());
+        return elements;
+    }
+
+    private Set<String> specifiedElements(WattsiSource source){
+        Set<String> elements = new HashSet<>();
+        for(ElementDefinition element : source.elementDefinitions()){
+            elements.add(element.name());
+        }
+        return elements;
+    }
+
+    @Test
+    @Ignore
+    // TODO restore this test once a policy has been determined for obsolete elements.
+    public void all_wattsi_elements_are_defined_in_the_code_generator() {
+        Set<String> generated = generatedElements();
+
+        List<String> undefined = specification.elementDefinitions().stream()
+            .filter(element -> !element.isObsolete())
+            .filter(element -> !generated.contains(element.name()))
+            .map(ElementDefinition::name)
+            .collect(toList());
+
+        assertEquals("HTML elements are missing", emptyList(), undefined);
+        // Currently missing (and mostly deprecated):
+        //   hgroup
+    }
+
+    @Test
+    public void only_wattsi_elements_are_defined_in_the_code_generator(){
+        Set<String> specified = specifiedElements(specification);
+
+        List<String> invalid = generatedElements().stream()
+            .filter(element -> !specified.contains(element))
+            .collect(toList());
+
+        assertEquals("HTML elements are invalid", emptyList(), invalid);
+    }
+
+}