Allow "sequential composition" to support composable `ExtraText`

[guard-who]

Now, we can't set different types (e.g, Static and Collapsible) of ExtraText in the configs like https://github.com/fmidue/modelling-tasks/blob/e46db2365b193e3e7c75179e5f3560777ca18d44/example/src/Modelling/CdOd/DifferentNames/Config.hs#L442 at the same time.

In order to add the support, here are some suggestions, one of which requires refactoring here. We should think about which one is more flexible to support the goal.

[jvoigtlaender]

Upon consideration, I think the best approach is to:

• drop NoExtraText from data ExtraText
• change the type of extra from OutputCapable m => ExtraText -> LangM m to OutputCapable m => [ExtraText] -> LangM m (and let it traverse the input list, stringing stuff together)
• replace all fields like extraText/addText :: ExtraText in client packages by ... :: [ExtraText] (and use [] as value where currently NoExtraText is used)