[Feature] Add symbol navigation commands

[pidgeon777]

My proposal is to implement two new commands:

• SymbolsOutlineNext: Navigate to the next symbol.

• SymbolsOutlinePrev: Navigate to the previous symbol.

Symbol navigation with those two commands should occur even if the Symbols Outline panel is not open.

Optionally: implement an highlight which appears for the newly selected symbol, which disappears (fade out?) after a certain user-defined time.

[rstacruz]

This would be great!

A totally imperfect solution would be something like this (already assumes symbols outline is open on the right-most pane):

nmap [s <c-w>bj<cr>
nmap ]s <c-w>bk<cr>

Just tried it out and I can see that this will be very useful.