Deprecate infix named args

[som-snytt]

Addresses #19072
Ports scala/scala#10857

[som-snytt]

Probably better as an external lint.

[odersky]

With named tuples this becomes more urgent. I agree, we should deprecate that syntax.

[mbovel]

Probably better as an external lint.

Looking at #21681, seems like this is common enough to require a compiler warning. Would you like to revive this PR? (Otherwise I am happy to do it.)

[som-snytt]

The warning at parser goes unseen in October because the arg is now a unituple. (I don't even know the lingo yet.) So the warning info is only meaningful as an addendum to the error here. The warning should be backported to LTS. I'll try that after a second coffee.

[som-snytt]

There is not much to addend, as it turns out. Mention the named arg, in case they didn't count the parens; handle the very contrived example where the param name and the tuple element name coincide.

[som-snytt]

Should be a migration warning (not deprecation); should not warn on named tuple ops. Probably move the warning to typer (as I meant to do) (or I was thinking refchecks, but it needs to see infix ops).