INSERT from SELECT cannot be parsed with an ON CONFLICT clause

[rtimush]

Parsing the following SQL

INSERT INTO foo
  SELECT 1, 'data'
  ON CONFLICT DO NOTHING

fails with

Expected: end of statement, found: CONFLICT at Line: 3, Column: 6

both for Generic and PostgreSQL dialects.

However, at least in PostgreSQL it seems to be a valid construct, equivalent to

INSERT INTO foo
  (SELECT 1, 'data')
  ON CONFLICT DO NOTHING

(this one is parsed by sqlparser correctly).

The same query without the "ON CONFLICT" clause can be successfully parsed too.