Translate COALESCE as ISNULL in more cases

[ranma42]

This is a continuation of #34171 that extends the conversion to handle all of the cases (and adds a couple of tests for corner-cases).

[ranma42]

Some additional notes: I tried to implement all of the suggestions by @roji and indeed they proved quite effective; OTOH the tests I added unveiled some additional limitations risks, which I mitigated with an approach the codebase is already taking elsewhere (basically widening the type).
I think it should be reasonable, but this might both be:

• insufficient in some cases (I only implemented the widening for 3 SQL types; I am unsure if other types might need a similar approach)
• it will result in different (SQL) types for the expressions

Specifically the changes I implemented include:

1. rely on StoreType to check if the types match
2. preserve the computed nullability
3. handle most of the types

The conversion now applies to all instances of COALESCE. Some limitation are:

• in several cases, unneeded Convert operations are inserted; in the case of sized types ([n]varchar, varbinary) they are almost always forced
• nullability is only propagated, not re-computed (I think if we want to have a relevant value here, we should compute it in the during the nullability processing and propagate it from there; this change is mostly to make sure the if/when we do that, the result is automatically forwarded as needed)

Even though it passes the testsuite, I think it might be a good idea to make it possible for users to opt-out, as I am afraid it might break some corner cases (especially around UDF and/or custom converters).