Scoped builder allows constructing invalid expressions applying LET/IN expressions

[shonfeder]

Afaik, the only support we have for something like lambda abstraction of operator in the Apalache IR is through let/in expressions like LET f(...) == ... IN f. When we need to construct an anonymous operator (e.g., in quint conversion), we get something that looks like this. However, it is not possible to apply an operator-expression of this form, as in (LET f(...) == ... IN f)(x). This has been discussed on #2483 and #2500.

I think one of the following should be done:

1. Add support for applying operator of this form
2. Catch and report these malformed expressions in the builder

(1) seems preferable and would make quint conversion easier, but I don't know how difficult it would be. (2) would at least lest us catch this inconvenience at the earliest opportunity, preventing an easy mistake.

WDYT @konnov and @Kukovec?

cc/ @gabrielamafra