Introduce triangle border

[nervo]

This looks like this :)

◤▼▼▼▼▼▼▼▼◥
▶        ◀
▶  Foo   ◀
▶  Bar   ◀
▶        ◀
◣▲▲▲▲▲▲▲▲◢

You could say that's a bit ugly, but such a border unleash its real power when used for styling - for instance - list elements:

  Foo
▶ Bar
  Baz

In this example, normal elements got a simple PaddingLeft(2), where focus element got PaddingLeft(1).BorderStyle(lipgloss.TriangleBorder()).BorderLeft(true).

[testinfected]

Exactly what I needed for some of my use cases. Thanks!

[maaslalani]

Hi @nervo, thanks for the PR and contribution! We haven't seen too much demand for the Triangle border and since we would like to keep the API surface as small as possible I think it is better if this border is added case-by-case into the code as a custom border rather than supporting it in Lip Gloss.

If many more people ask for this border however we would gladly support it and I will re-open this PR!

Thanks again for the awesome contribution!

[maaslalani]

For what it's worth, we are planning on adding a List renderer to Lip Gloss which should solve many of your use cases of this border.

• List Renderer #240