Allow with expr AS ident syntax in WITH clause

[MawiraIke]

This is to allow for ClickHouse WITH clause variants.
See #428

[seancorfield]

This still has references to ClickHouse. It should not. It also has no tests and no documentation updates. It also makes gratuitous indentation/whitespace changes.

It should be based purely on the types of the two parts of the WITH clause (whether they are ident?).

If you want to submit an updated PR with the above changes, feel free, but I'm happy to fix #433 myself when I get some time.

[MawiraIke]

Okay, let me push an updated PR. I assumed you might have wanted to retain the current functionality for existing dialects. I.e throwing an error when the expression is not a subquery expression since the syntax is not supported anyway. I am updating the code to change the syntax if the expr is ident?

[MawiraIke]

@seancorfield I have updated the code with the suggested changes. I am available to make any further changes.
Also, I could not base it on ident? only and had to had string? because there is no simpler way to represent strings with spaces without further code changes.

[seancorfield]

Thanks. I've kicked off a CI run and if it passes, I'll merge this in. Much appreciated!