fix(VTreeview): avoid inaccessible items when overflow

[J-Sek]

Description

Tree view gets horizontal overflow and shows the items. It is not easy to guess how to work around this issue. It would be great if people would not need to resort to for CSS tricks. The idea is that they can inspect the title, see 40px for min-width and eventually find a Sass variable or override it with CSS.

There is a side-effect of pushing append-icon's to the right. I don't think anyone would really mind it as it only occurs when the nesting is so deep the whole thing is already in a bad shape.

Feel free to switch v-treeview to v-list on the examples linked below.

resolves #21387

Markup:

Playgrounds with a toggle to see before/after:

• deep nesting
• long title & subtitle