Updated Language Evolution for chrono descending, plus clean up

[atsansone]

This PR updates the Language Evolution page. I made some language updates, but re-sorted the versions as reverse chronological and added release dates and links to the release announcements.

[atsansone]

Staged at https://dart--staging.web.app/guides/language/evolution

[parlough]

Thanks for preparing these changes!

I've gone through and left some issues, suggestions, and nits. Please feel free to push back or discuss anything.

One main thing is I think this doc really should only be focused on historical language changes and features, it's not a page for release notes. We should avoid adding SDK changes to be consistent and likely remove the few that exist already.

[atsansone]

@parlough : Merging my changes after this review.

[parlough]

Thanks for making some adjustments and responding to my comments and questions!

Some of these review comments are responses to previous ones, so you'll need to look back at unresolved comments from the last review.

I'm also happy to discuss synchronously any of the concerns I raised if that works better for you as well.

[parlough]

This reverts to an older version of site-shared, please undo this change.

There's a few ways to do this. Two of these easiest might be:

• git submodule foreach git pull origin main - Updates each submodule to track their latest commit on main. If you named dart-lang/site-www something besides origin, replace it with that.
• Switch to a branch tracking the latest origin/main and run git submodule update there.

[parlough]

It's getting super close. Thanks for all of those changes.

A few last comments and answers to questions:

[parlough]

The <: should be outside of the code.

Suggested change

	Constraints of the forms similar to `Null <:FutureOr` now yield `Null`
	Constraints of forms similar to `Null` <: `FutureOr` now yield `Null`

Also we might want to avoid the symbol completely. It's something readers may not be familiar with. Perhaps something like?

Suggested change

	Constraints of the forms similar to `Null <:FutureOr` now yield `Null`
	Constraints of the form where `Null` is a subtype of `FutureOr` now yield `Null`

[parlough]

I had to rebase these changes to fix some issues with the PR and pushed some minor changes. There's two last changes that need to be addressed before merging but I want you to take a look.

Otherwise, looks good to me. Thanks for doing this and all of the discussion. I think the page has got to a really great spot and has a clearer vision now.

Optionally get a second review from someone after making those fixes, but since it's mostly reorganized and rewritten existing content, it should be fine.

[parlough]

I think the "Constraints of the forms similar to" portion is pretty confusing, can we simplify it? I also didn't notice the missing generic on FutureOr<T>. That's important for knowing what the following T is referring to.

Suggested change

	Constraints of the forms similar to `Null` serves as a subtype of `FutureOr`
	now yield `Null` as the solution for `T`.
	Constraints where `Null` serves as a subtype of `FutureOr<T>`
	now yield `Null` as the solution for `T`.

[parlough]

This indicates it uses the 2.18 language version by default, not later. It can't use later language versions or else that would break the constraint.

Suggested change

	indicates that this package uses the Dart 2.18 language version or later.
	indicates that this package uses the Dart 2.18 language version by default.