fix(completions): complete right columns right after JOIN ON

[juleswritescode]

Before, we were only completing columns in a JOIN in a "Binary Expression".
But for TreeSitter to recognize a Binary Expression, we need an = sign.

So this wouldn't complete, since there's no = sign:
select * from users u join posts p on u.{}

Now, we're checking for the ON node, and completing columns if we're after it.

Note: Multiple joins are each individual join clauses with their own separate ON nodes, so there's no intereference:

select * 
from 
users u
join posts p on u.id = post.user_id -- join clause 1
join {} -- join clause 2

We won't autocompete a column, since there's no ON node in join cluase 2 🥳

[juleswritescode]

we might wanna fix this upstream at some point