inference: Add `widen_call_result` interface
Extract the widening-to-`Any` decision applied to call return types at
unused call sites into a new
```julia
widen_call_result(interp::AbstractInterpreter, si::StmtInfo, state::CallInferenceState, sv::AbsIntState)
```
interface, mirroring the style of `bail_out_call` and friends.
The default implementation preserves existing behavior:
```julia
call_result_unused(si) && !(state.rettype === Bottom)
```
This is motivated by use cases such as LSP servers (esp. JETLS), where
the widened `Any` shown at unused call sites is misleading to users —
for those interpreters, holding the precise inferred return type is more
useful. Such consumers can now opt out of (or generalize) the widening
by overloading this method without forking the call inference pipeline.
