feat: Better provider info and fix LSP deprecated warnings

- 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)

lua/outline/providers/markdown.lua

@@ -1,9 +1,20 @@
+-- NOTE
 -- Our own markdown provider is used because legacy symbols-outline considered
 -- the case where markdown does not have an LSP. However, it does, so as of now
 -- this module is kept for use when user opens symbols outline before the
--- markdown LSP is ready. Please also see comment in providers/init.lua
+-- markdown LSP is ready.
+--
+-- On buffer open the LSP may not be attached immediately. Before the LSP is
+-- ready if the user opens the outline, our own markdown provider will be used.
+-- After refreshing/reopening, the provider will then switch to the LSP (if the
+-- user has a markdown LSP). That is, if the user has an applicable markdown LSP.
+--
+-- If they don't this provider will always work as usual.
+
+local M = {
+  name = 'markdown',
+}
 
-local M = {}
 
 ---@return boolean ft_is_markdown
 function M.should_use_provider(bufnr)