Changing exa from grid to tree (preview-tui)

[manceras]

Hello,
in the plugin preview-tui the prefered program to preview is tree, if tree is not available, the second option is exa. As exa has a feature to list a directory in a tree, wouldn't be more consistant showing it that way?
It would be a very easy change, just changing the the flag from -G to -T.

[jarun]

Sounds reasonable. Please raise the PR.