Add diagnostics for double semicolon at short lambda

This change introduce diagnostics when there is a double semicolon at
the end of the short function body for unnamed function:
```cpp
  l2 := :() -> int = 24;;
```
Generates:
```
error: a short function syntax shall end with a single semicolon (at ';')
```

Author: filipsajdak

source/parse.h

@@ -3010,6 +3010,12 @@ class parser
                     next();
                     return {};
                 }
+                if ( // check if a single-expression function is followed by an extra second semicolon
+                    decl->initializer && decl->initializer->is_expression()
+                    && !done() && curr().type() == lexeme::Semicolon
+                ) {
+                    error("a single-expression function should end with a single semicolon");
+                }
                 if (!(*func)->contracts.empty()) {
                     error("an unnamed function at expression scope currently cannot have contracts");
                     next();