Add Sealed trait pattern

[rbran]

No description provided.

[simonsan]

I like it, I skimmed over it and it looks like a good addition to this repository. Maybe others, e.g. @neithernut and/or @pickfire can give it a read as well to leave some always valued feedback. So we can merge it next week.

[neithernut]

LGTM, but I feel it could be extended a bit more:

• Sealed traits in rust are rather nieche, you don't usually use them. Well, unless you do. Imo this is one of those more "advanced" topics.
• The example used (OsStrExt) may already be viewed as something of a special case, as it exists multiple types for different configurations with required methods that may or may not diverge from each other: unix and WASI are, on the surface, equivalent; windows, as always, differs. You do list those declarations, but imo that may just confuse some readers.
• At the same time that very example also hints at an additional motivation for sealed traits: ensuring comparability not only between versions (if you are not capable or willing to come up with a stable interface) but also between multiple configurations.

[simonsan]

@rbran Could you please rebase on main, we just merged our refactoring efforts to support translations. Basically SUMMARY.md and the other markdown files are moved into ./src/**/{file}. Cheers.