feat(symbols): add wide and tall proportional border set

[Emivvvvv]

Adds PROPORTIONAL_WIDE and PROPORTIONAL_TALL border sets.

symbols::border::PROPORTIONAL_WIDE

▄▄▄▄
█xx█
█xx█
▀▀▀▀

symbols::border::PROPORTIONAL_TALL

█▀▀█
█xx█
█xx█
█▄▄█

Fixes: #834

[joshka]

It's convention to leave a blank line between the title of the comment and the body.

It would be nice to define proportional here as visually equal width and height.

I think the name is right.

Thanks for this.

[Emivvvvv]

How about now? And you're welcome but I think I'm the one who should thank because you and the other maintainers are providing the best environment for open-source beginners like me :)

[joshka]

LGTM. Approving. Will wait on another maintainer to confirm the name makes sense to them as well.

[joshka]

Reformatted first comment for neater changelog.

[kdheepak]

I think the name is good, it has the same meaning to the naming in Padding. There's Contraints::Proportional that we might want to consider renaming to minimize confusion.

[joshka]

Thanks for the PR @Emivvvvv :)