add missing gson library for LSP (see #564)

java/build.xml

@@ -53,10 +53,11 @@
   <fileset id="lsp4j.files" dir="mode">
     <include name="org.eclipse.lsp4j.jar" />
     <include name="org.eclipse.lsp4j.jsonrpc.jar" />
+    <include name="gson.jar" />
   </fileset>
 
   <condition property="lsp4j.present">
-    <resourcecount refid="lsp4j.files" when="eq" count="2" />
+    <resourcecount refid="lsp4j.files" when="eq" count="3" />
   </condition>
 
   <!-- ok to ignore failed downloads if we at least have a version that's local -->
@@ -79,6 +80,11 @@
          dest="mode/org.eclipse.lsp4j.jsonrpc.jar"
          ignoreerrors="${lsp4j.ignorable}"
          usetimestamp="true" />
+
+    <get src="https://repo1.maven.org/maven2/com/google/code/gson/gson/2.9.1/gson-2.9.1.jar"
+         dest="mode/gson.jar"
+         ignoreerrors="${lsp4j.ignorable}"
+         usetimestamp="true" />
   </target>
 
   <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->

java/mode/.gitignore

@@ -1,4 +1,5 @@
 JavaMode.jar
 org.eclipse.lsp4j.jar
 org.eclipse.lsp4j.jsonrpc.jar
+gson.jar
 

java/processing4-java.iml

@@ -188,5 +188,14 @@
         <SOURCES />
       </library>
     </orderEntry>
+    <orderEntry type="module-library">
+      <library name="gson">
+        <CLASSES>
+          <root url="jar://$MODULE_DIR$/mode/gson.jar!/" />
+        </CLASSES>
+        <JAVADOC />
+        <SOURCES />
+      </library>
+    </orderEntry>
   </component>
 </module>