refact: mark the old LanguageClientImpl as deprecated for removal

Author: rubenporras

org.eclipse.lsp4e/src/org/eclipse/lsp4e/LanguageClientImpl.java

@@ -0,0 +1,15 @@
+package org.eclipse.lsp4e;
+
+import org.eclipse.lsp4e.client.DefaultLanguageClient;
+
+/**
+ * This class is deprecated, and only present to avoid breaking
+ * existing code due to the refactor done to make the previous
+ * class officially available as API.
+ * <p>
+ * Use {@link DefaultLanguageClient} instead.
+ */
+@Deprecated(forRemoval = true)
+public class LanguageClientImpl extends DefaultLanguageClient {
+
+}