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`.
Primarily:
- Utils
- Sidebar (removed the need of writer.lua)
- Resolve keymaps shortcut in config eraly
- Put highlight functions into highlight.lua
- Put functions that do stuff on outline window into view.lua
- Provider priorities can now be configured through `providers.priority`
- Each provider can have a get_status() function that returns a string
for its status. For LSP it returns the client name.
- :OutlineStatus logic refactored, together with provider checking
functions in `providers/init.lua`
- Switch from vim.lsp.buf_get_clients to vim.lsp.get_active_clients
(former was deprecated)
- Fixed a careless mistake from symbols-outline that seems to be an
unreachable bug (lsp)
