[Feature] Focus the Symbols Outline panel

[pidgeon777]

My proposal is to update the SymbolsOutlineOpen command so that it focuses on the Symbols Outline panel currently attached to the buffer if it is already open.

Or, add a new SymbolsOutlineFocus command which does the same thing.

[hedyhli]

Hi, for the time being (where simrat39 is not very active) I'm maintaining a fork where I've implemented this feature and merged some other PRs -- see the focus_on_open setting in readme.

Also do note on the fork status in the readme, feel free to try it if you really want this feature but proceed with caution :)