LibSQL: Implement a DESCRIBE TABLE statement

This statement (for now) outputs the name and types of the different
attributes in a table. It's not standard SQL but all DBMSs that I know
of implement a sort of statement for such functionality.

Since the output of DESCRIBE TABLE is just a relation, an internal
schema, `master` was created and a table definition for DESCRIBE into
it. The table definition and the master schema are not accessible by the
user.

Userland/Libraries/LibSQL/SQLResult.h

@@ -19,6 +19,7 @@ namespace SQL {
 #define ENUMERATE_SQL_COMMANDS(S) \
     S(Create)                     \
     S(Delete)                     \
+    S(Describe)                   \
     S(Insert)                     \
     S(Select)                     \
     S(Update)