LibSQL: Parse and execute sequential placeholder values

This partially implements SQLite's bind-parameter expression to support
indicating placeholder values in a SQL statement. For example:

    INSERT INTO table VALUES (42, ?);

In the above statement, the '?' identifier is a placeholder. This will
allow clients to compile statements a single time while running those
statements any number of times with different placeholder values.

Further, this will help mitigate SQL injection attacks.

Userland/Libraries/LibSQL/AST/Expression.cpp

@@ -29,6 +29,13 @@ ResultOr<Value> NullLiteral::evaluate(ExecutionContext&) const
     return Value {};
 }
 
+ResultOr<Value> Placeholder::evaluate(ExecutionContext& context) const
+{
+    if (parameter_index() >= context.placeholder_values.size())
+        return Result { SQLCommand::Unknown, SQLErrorCode::InvalidNumberOfPlaceholderValues };
+    return context.placeholder_values[parameter_index()];
+}
+
 ResultOr<Value> NestedExpression::evaluate(ExecutionContext& context) const
 {
     return expression()->evaluate(context);