[ui/ux][tree] Help the tree indentation with vertical lines, mimic VS Code

[kittaakos]

Description

From time to time, it is hard to identify the indentation in the tree. We used to have a larger indentation, now we have a small one to adjust Theia's styles to VS Code. Besides using small indentations, VS Code also shows vertical lines in the tree which is cool. Let's do the same.

See the attached screenshot:

and the :hover behavior too:

Reproduction Steps

OS and Theia version:

Diagnostics: