Features' requests for symbols' tree: follow mode support + settings for folding depth on startup

[CptnGreen]

Hello fellow developers!

Enjoing your great package!

Missing some features though, here they are:

1. treemacs supports peek/follow mode (P in spacemacs) when you navigate inside the tree and buffer follows files in focus, would be handy to have this for symbols' tree too

2. symbols' tree opens fully unfolded by default, would be nice to have some setting to change this (say, to set startup folding depth)

Thanks for your attention)