Add MySQL 8.4 context.

Author: niconoe-

psalm-baseline.xml

@@ -656,6 +656,9 @@
   <file src="src/Contexts/ContextMySql80300.php">
     <PropertyTypeCoercion occurrences="1"/>
   </file>
+  <file src="src/Contexts/ContextMySql80400.php">
+    <PropertyTypeCoercion occurrences="1"/>
+  </file>
   <file src="src/Lexer.php">
     <LoopInvalidation occurrences="3">
       <code>$this-&gt;last</code>