SELECT DISTINCT syntax no longer works

[lnhrdt]

SQLDelight Version

2.0.2

SQLDelight Dialect

postgresql

Describe the Bug

I used to be able to write syntax like SELECT DISTINCT col1, col2 FROM ... but since the introduction of DISTINCT ON I can only write SELECT DISTINCT ON (col1, col2) col1, col2 FROM ....

I would prefer to keep both syntaxes to not break migrations that were written before the DISTINCT ON syntax was introduced.

This is a followup from #4584 (comment)

Stacktrace

compile errors from the plugin of the form:

`ON expected, got 'col1'`

[griffio]

I will create a PR as it looks like the grammar needs a small change to fix it 🔨

Also tests are needed for the basic SELECT DISTINCT as there doesn't appear to be any in the base sql-psi grammar.