languaget: mark non-sensical default implementations = 0

Every language must override these methods.

Author: tautschnig

src/json-symtab-language/json_symtab_language.h

@@ -36,6 +36,15 @@ class json_symtab_languaget : public languaget
     UNIMPLEMENTED;
   }
 
+  std::string id() const override
+  {
+    return "json_symtab";
+  }
+  std::string description() const override
+  {
+    return "JSON symbol table";
+  }
+
   std::set<std::string> extensions() const override
   {
     return {"json_symtab"};

src/langapi/language.h

@@ -125,10 +125,9 @@ class languaget:public messaget
 
   // language id / description
 
-  virtual std::string id() const { return ""; }
-  virtual std::string description() const { return ""; }
-  virtual std::set<std::string> extensions() const
-  { return std::set<std::string>(); }
+  virtual std::string id() const = 0;
+  virtual std::string description() const = 0;
+  virtual std::set<std::string> extensions() const = 0;
 
   // show parse tree