feat(tree): support width and indenter styling

[dlvhdr]

Some of the stuff I've been implementing on charmbracelet/bubbles#639 should go into lipgloss. This would help simplify the bubble code and is better for separating the rendering logic into lipgloss itself.

This PR adds two things:

1. The ability to style the indenter with both a static style and a function style
2. The ability to specify the tree's width, which allows tree nodes to stretch to fill it.

I've also wrote a new example to show what this enables.

TODO: check this against my tree bubble

Warning

This does introduce a "breaking change", not in the API but:
Trees that have subtrees with their own renderers, meaning they set a custom item/enumerator style, will need to now also set an indenter style, if they want to keep a consistent indentation styling.
If they don't set one the default empty style is used and it can look weird to suddenly have 2 kinds of indentation stylings.
Previously this was solved by using the enumerator style.