Tidy: Enforce that groups in declare_feature! are sorted by version number

[Centril]

In the declare_features! ( ... ) list in src/libsyntax/feature_gate.rs, enforce that groups (internal, user-facing, ...) of features are ordered by version number with the newest last. The groups will need to be scoped in some way with a comment, e.g. // feature group START + feature group END.

This makes it easier to see what things are newer and gives a better overview when considering stabilization.

cc @oli-obk
cc #60354
cc #60362

[rasendubi]

I would like to work on this. I think I'll be able to add the feature to tidy, but I'll need help for splitting features into groups.