Use clause parse error reporting

[kubukoz]

Following changes from #119 (or maybe it was the case even before), error reporting in use clauses is now pretty confusing:

Same goes for qualified operation names:

I think it's most likely caused by the backtracking in .soft (so it would've been like this even before #119 after all). A proper fix would probably involve removing the backtracking in Parser[QualifiedIdentifier]:

https://github.com/kubukoz/smithy-playground/blob/main/modules/parser/src/main/scala/playground/smithyql/parser/Parser.scala#L150-L167

and replacing that parser's occurrences with something more usecase-driven (i.e. in operation names, read one segment and then decide what we're actually parsing).