Add diagnostics for empty statements (extra semicolon)

This change introduce diagnostics when there is a semicolon
without statement
```cpp
;
```
Generates:
```
error: empty statement is not allowed - remove extra semicolon (at ';')
```
That also handles cases when there is double semicolon after a proper
statemnt:
```cpp
  i := 0;;
```
Generates:
```
error: empty statement is not allowed - remove extra semicolon (at ';')
```

Author: filipsajdak

source/parse.h

@@ -4394,6 +4394,11 @@ class parser
     )
         -> std::unique_ptr<statement_node>
     {
+        if (!done() && curr().type() == lexeme::Semicolon) {
+            error("empty statement is not allowed - remove extra semicolon");
+            return {};
+        }
+
         auto n = std::make_unique<statement_node>();
 
         //  Now handle the rest of the statement