[Dart 3] Class modifiers

[MaryaBelanger]

Reviewers:

• New class modifiers page staged here
• Minor changes to mixin page here
• Added modifiers to Keywords page
• Moved "Abstract classes" section off of "Classes" page and to "Class modifiers > abstract" section (didn't update text) here

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Updates in progress after first review:

• Need to change the examples so the code where extends, implements, etc is failing is clearly NOT in the same library where the modified class is declared.
• The way I have it written now makes it seem like it's in the same library, in which case the modifiers wouldn't restrict anything.
• Call out that the modifiers affect outside libraries more often and explicitly.
• Need to change the first line here: "Dart has no interface keyword."

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TODO (possibly separate PRs):

• Change code to actual code excerpts
• Address changes to core libraries in library tour ([Dart 3] Document general use of class modifier by Dart core libraries in Library Tour #4730)
• In /language/class-modifiers#sealed, need to change link on "exhaustiveness checking" to actual section once that's written

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Fixes #4497
Fixes #4662
Fixes #4581

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[eernstg]

Added a couple of extra comments.

[munificent]

This looks great!

[atsansone]

@MaryaBelanger : Solid work. Issues are:

• Condition, then instruction per GSG.
• Write in active voice per GSG.
• Avoid parentheses per GSG.

I provided suggestions throughout.

[atsansone]

Great update! A few issues, suggestions, and nits. Handle the issues and no need for another pass.