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/types/outline.lua

@@ -65,11 +65,13 @@
 -- PROVIDER
 
 ---@class outline.Provider
----@field should_use_provider fun(bufnr:integer):boolean
----@field hover_info fun(bufnr:integer, params:table, on_info:function)
----@field request_symbols fun(on_symbols:function, opts:table)
 ---@field name string
 ---@field get_status? fun():string[]
+---@field supports_buffer fun(bufnr:integer):boolean
+---@field request_symbols fun(on_symbols:function, opts:table?)
+---@field hover_info? fun(bufnr:integer, params:table, on_info:function)
+---@field rename_symbol? fun(sidebar:outline.Sidebar)
+---@field code_actions? fun(sidebar:outline.Sidebar)
 
 -- HELP