fix: add an error message when a combined grammar has no implicit lexer grammar (fixes #687)

Author: bjansen

src/main/java/org/antlr/intellij/plugin/parsing/ParsingUtils.java

@@ -310,6 +310,9 @@ public static Grammar[] loadGrammars(VirtualFile grammarFile, Project project) {
 			case ANTLRParser.COMBINED :
 				lg = g.getImplicitLexer();
 				if ( lg==null ) {
+					String msg = "No implicit lexer grammar found in combined grammar " + g.name
+						+ ". Did you mean to declare a `parser grammar` instead?\n";
+					console.print(msg, ConsoleViewContentType.ERROR_OUTPUT);
 					lg = BAD_LEXER_GRAMMAR;
 				}
 				ANTLRv4PluginController.LOG.info("loadGrammars combined: "+lg.name+", "+g.name);

src/main/resources/META-INF/plugin.xml

@@ -74,6 +74,7 @@ For really big files and slow grammars, there is an appreciable delay when displ
   <change-notes><![CDATA[
 	<ul>
 	    <li>Fixed syntax highlighting in 2024.1 EAP (<a href="https://github.com/antlr/intellij-plugin-v4/issues/691">#691</a>)</li>
+	    <li>Explain when a grammar might lack the `parser` keyword (<a href="https://github.com/antlr/intellij-plugin-v4/issues/687">#687</a>)</li>
     </ul>
 	 See the complete list of <a href="https://github.com/antlr/intellij-plugin-v4/issues?q=milestone%3A1.23.1fix+is%3Aclosed">fixed issues</a>.
       ]]>