[v12] Update some things

[DavidBruchmann]

Implementation of TypoScript, PageTS, and probably further things are old-style.
While some things should be moved into Configuration/Overrides/... others like code for implementation of PageTS can be dropped if the file Configuration/page.tsconfig is created as main file for the whole PageTS.

Some changes can certainly be back-ported for v11.

[DavidBruchmann]

An extension check in the Installtools shows perhaps some more things to update.

[DavidBruchmann]

replace INCLUDE_TYPOSCRIPT by @import syntax

[DavidBruchmann]

All changes together make an update for v13 easier, while they are working already in v12 and v11.

[tdeuling]

@DavidBruchmann thanks for reporting. Feel free to create a PR 🙂 Since I've a huge workload + vaction in next weeks, I'm not able to support.

[kaystrobach]

Will take a look. Also already took a deep look into the new site configuration features and hope to use these in the future.