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/Token.h

@@ -171,6 +171,7 @@ namespace SQL::AST {
     __ENUMERATE_SQL_TOKEN("_blob_", BlobLiteral, Blob)                    \
     __ENUMERATE_SQL_TOKEN("_eof_", Eof, Invalid)                          \
     __ENUMERATE_SQL_TOKEN("_invalid_", Invalid, Invalid)                  \
+    __ENUMERATE_SQL_TOKEN("?", Placeholder, Operator)                     \
     __ENUMERATE_SQL_TOKEN("&", Ampersand, Operator)                       \
     __ENUMERATE_SQL_TOKEN("*", Asterisk, Operator)                        \
     __ENUMERATE_SQL_TOKEN(",", Comma, Punctuation)                        \