refactor(qc): remove `ScalarCondition::[Not]InTemplate`
Remove `ScalarCondition::InTemplate` and `ScalarCondition::NotInTemplate`
and instead add placeholders as the third variant in
`ConditionListValue`. This simplifies the implementation and makes
some illegal states unrepresentable (previously it was possible to
construct `ScalarCondition::InTemplate`/`ScalarCondition::NotInTemplate`
with arbitrary `PrismaValue`s and with field references).
