Fix $LINES and $COLUMNS in fzf previews

[MaienM]

The $LINES and $COLUMNS environment variables are not set correctly in FZF previews. This is a known issue, and it is being worked on. Once this is resolves, I should make sure that the previews are modified in such a way that these are applied correctly.

[MaienM]

The cause of this is that FZF runs the previews in $SHELL, and zsh doesn't use $LINES and $COLUMNS from the environment. As a workaround, FZF now provides $FZF_PREVIEW_LINES and $FZF_PREVIEW_COLUMNS.