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/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(SOURCES
     AST/CreateSchema.cpp
     AST/CreateTable.cpp
+    AST/Describe.cpp
     AST/Expression.cpp
     AST/Insert.cpp
     AST/Lexer.cpp