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.

Tests/LibSQL/TestSqlExpressionParser.cpp

@@ -131,6 +131,19 @@ TEST_CASE(null_literal)
     validate("NULL"sv);
 }
 
+TEST_CASE(bind_parameter)
+{
+    auto validate = [](StringView sql) {
+        auto result = parse(sql);
+        EXPECT(!result.is_error());
+
+        auto expression = result.release_value();
+        EXPECT(is<SQL::AST::Placeholder>(*expression));
+    };
+
+    validate("?"sv);
+}
+
 TEST_CASE(column_name)
 {
     EXPECT(parse(".column_name"sv).is_error());