orm: add unique fields & add drop table stmt (#9684)

vlib/orm/README.md

@@ -4,7 +4,9 @@
 
 ### Fields
 
-- `[primary]` set the field as the primary key
+- `[primary]` sets the field as the primary key
+- `[unique]` sets the field as unique
+- `[unique: 'foo']` adds the field to a unique group
 - `[nonull]` field will be `NOT NULL` in table creation
 - `[skip]` field will be skipped
 - `[sql: type]` sets the type which is used in sql (special type `serial`)
@@ -26,6 +28,14 @@ sql db {
 }
 ```
 
+### Drop
+
+```v ignore
+sql db {
+    drop table Foo
+}
+```
+
 ### Insert
 
 ```v ignore