Scrolling Bufferline

[CedricMeu]

This PR adds a scrolling bufferline.
Previously, anything overflown to the right wouldn’t be shown.
Now the active buffer tab will be scrolled as much to the center of the bar as possible, creating a scrolling effect.

[the-mikedavis]

Rather than making the rendering configurable I would prefer we be opinionated and pick a sane default and have no config. IMO scroll is the best presentation, plus it aligns with other UIs like browsers.

[CedricMeu]

Rather than making the rendering configurable I would prefer we be opinionated and pick a sane default and have no config. IMO scroll is the best presentation, plus it aligns with other UIs like browsers.

Like I mentioned, I am in favour of using scroll as the default.
Also overflow could be removed as no sane person would favour that over scroll nor wrap.
I am however hesitant on removing the configuration for the behaviour as other people might like wrapping over scrolling.
If we remove the option, someone down the line will probably reimplement it.

[pascalkuthe]

as @the-mikedavis said we usually implement a single opiniated style in favour of adding a bunch of options. If somebody implements a second style in the future we simply won't accept the PR.

[CedricMeu]

As requested, I removed configurability for the bufferline behaviour. The bufferline now only scrolls when overflown.

[dvdvgt]

Hi all! Thanks for your hard work on helix. May I kindly ask if there's any update regarding this PR?