feat: Support IF NOT EXISTS on CREATE TYPE

[jzaralim]

Description

Added support for new syntax: CREATE TYPE [IF NOT EXISTS] foo AS bar. Addresses #5991

If IF NOT EXISTS is included in the query, then creating a type that already exists will return the following message:

 Message
--------------------------------------------
 FOO is already registered with type STRING
--------------------------------------------

If not, then this error will be thrown:

Cannot register custom type 'WOW' since it is already registered with type: STRING

Testing done

Added unit tests and manually tested

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[spena]

LGTM! Thanks