feat(viewport): add SetLines method in Viewport

[semihbkgr]

In certain use-cases I encountered, setting content by lines proved to be more advantageous both in terms of functionality and performance.

[maaslalani]

I think this makes sense to me. What do you think @meowgorithm?

[meowgorithm]

This is fairly off the cuff, but I wonder if SetContent would be better off accepting variadic args rather than increasing surface area of the API here.

func (m *Model) SetContent(s string) // before
func (m *Model) SetContent(line ...string) // after

[semihbkgr]

@meowgorithm do you mean that the API should accept only lines instead of whole content?

I think it will break the backward-compatibility for anyone using the Viewport model and it may not be suitable for every use case.

[meowgorithm]

I actually mean that SetContent would be able to accept multiple args, joining them with newlines as a convenience. This would be compatible with the current API.

vp := viewport.New(80, 40)
viewport.SetContent("a\nb", "c")
fmt.Println(viewport.View()) // outputs "a\nb\nc"