Prefix all statements with S

For classes that mirror reserved keywords, a prefix of S was needed.
Adding the same prefix for the others for consistency sake.

src/Grammar/Statements/Expression.php → src/Grammar/Statements/SExpression.php

@@ -5,7 +5,7 @@
 use Waffle\Grammar\Expr;
 use Waffle\Grammar\Stmt;
 
-class Expression extends Stmt
+class SExpression extends Stmt
 {
  public function __construct(
  private readonly Expr $expression

src/Grammar/Statements/Visitor.php

@@ -8,7 +8,7 @@ public function visitBlockStmt(Block $stmt);
 
  public function visitClassStmt(SClass $stmt);
 
- public function visitExpressionStmt(Expression $stmt);
+ public function visitExpressionStmt(SExpression $stmt);
 
  public function visitFunctionStmt(SFunction $stmt);