add "crowded" and "dense-in-itself" aliases

[StevenClontz]

Several talks at STDC 2026 have used these terminologies, and I realized we don't have them in our property list.

Could probably use better reference than @prabau's random Math SE post.

[prabau]

How about adding a paragraph at the end like this:
*Note*: A space that does not satisfy {P139}, i.e., without isolated point, is called "dense-in-itself" or "crowded". See {{wikipedia:Dense-in-itself}} and {{mathse:3823020}}.

and also add that wikipedia article to the refs: list.

[StevenClontz]

Thanks, done more or less as you suggested.