Do not suggest using | in let assignments

[estebank]

Fix #61573.

r? @petrochenkov

[petrochenkov]

I'd prefer not to do this, but rather address the "a | b won't type-check in any case" / a | b would still not be a legal pattern using a simple heuristic.

If we have a comma separated list PAT, PAT, PAT, parse it until the end, then decide how to interpret it.

• Collect a set of lower-case identifiers for each PAT (with a small visitor implementing only visit_ident).
• If those sets are identical, recommend only PAT | PAT | PAT.
• If none of those sets intersect with each other, recommend only (PAT, PAT, PAT).
• Otherwise recommend both and let the user disambiguate.

This work is done only in the error case, so it's not a performance issue, and the heuristic would be useful for all positions, not only let.

[estebank]

@petrochenkov I like the idea, but it'll take me a while making the time to implement it.