support parser "drop table"

Author: sjjian

README.md

@@ -14,6 +14,8 @@ this is an oracle sql parser. ref: https://docs.oracle.com/en/database/oracle/or
 |Alter table|Drop constraint| :heavy_check_mark:| |
 |Create table|Relational table|:heavy_check_mark:|:heavy_check_mark:|
 |Create index|Relational table|:heavy_check_mark:| |
+|Drop table|-|:heavy_check_mark:|:heavy_check_mark:|
+
 ## usage
 ```go
 package main

ast/ddl.go

@@ -108,10 +108,19 @@ type OutOfLineConstraint struct {
 }
 
 /*
-	Create Table  Statement
+	Create Index  Statement
 	see: https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/CREATE-INDEX.html#GUID-1F89BBC0-825F-4215-AF71-7588E31D8BFE
 */
 
 type CreateIndexStmt struct {
 	node
 }
+
+/*
+	Drop Table  Statement
+	see: https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/CREATE-INDEX.html#GUID-1F89BBC0-825F-4215-AF71-7588E31D8BFE
+*/
+type DropTableStmt struct {
+	node
+	TableName *TableName
+}

lexer.go

@@ -301,6 +301,7 @@ var keywordMap = map[string]int{
 	"usable":                  _usable,
 	"unusable":                _unusable,
 	"invalidation":            _invalidation,
+	"purge":                   _purge,
 }
 
 func init() {