Can be disruptive if you've got many windows with buffers across
different projects. Also, SPC o P is available to "jump to current file"
in the treemacs window.
This has proven intrusive, especially when you have files from different
projects open and you switch between them rapidly. Enabling
treemacs-follow-mode is left up to the user. Otherwise, the SPC o P
keybind and treemacs-find-file command can this manually and on-demand.
Also remove hide-mode-line-mode from treemacs because it's redundant
with the doom-themes treemacs icon theme.
Alexander-Miller/treemacs#212 introduced treemacs-persist-file, which we
now use instead of tools/treemacs' old treemacs-persistence hack (which
didn't work).
Relevant to #669
