refactor: Provider symbol actions

It makes sense to store the provider each sidebar is using with own
self.provider fields, no way that did not occur to me before this.

The old `_G._outline_current_provider` ironically, can now be replaced
by `require('outline').current.provider`.

lua/outline/providers/coc.lua

@@ -28,7 +28,7 @@ end
 ---@param result table
 local function convert_symbols(result)
   local s = {}
-  local kinds_index = require('outline.symbol').str_to_kind
+  local kinds_index = require('outline.symbols').str_to_kind
   -- rebuild coc.nvim symbol list hierarchy according to the 'level' key
   for _, value in pairs(result) do
     value.children = {}