feat(viewport): gutter column, soft wrap, search highlight

[caarlos0]

This PR adds several somewhat related features to viewport.

Highlights

First, is highlights! This should allow users to add search features to their tea apps. You can see an example of this here.

CleanShot.2025-01-23.at.13.39.33.mp4

The highlights take into account visible characters and ANSI escape sequences, so users should be able to do something like:

m.SetHighlights(re.FindAllStringIndex(m.GetContent(), -1)

And it should take care of everything.

Soft wrapping

Previously, users would have to soft wrap on their side, which is not as simple as it looks some times.

Having it here also helps with the left gutter feature, oops, I'm getting ahead of myself 🙈

Users can m.SoftWrap = true to enable this feature.
You can see soft wrapping in action in the video above.

Left Gutter

This allows to add a fixed gutter to the left side of the viewport. You can use these to show line numbers, indicators, and more.

Users can set the gutter function with m.LeftGutterFunc = <your func>.

Example with line numbers:

CleanShot.2025-01-23.at.13.40.51.mp4

Example with a selector:

CleanShot.2025-01-23.at.13.42.40.mp4

[bashbunni]

TODO: create a runnable example with the API changes @bashbunni

[caarlos0]

TODO: create a runnable example with the API changes @bashbunni

On this branch of gum:

cat README.md | go run . pager

Working on making it work in gum filter, too