refactor: Add a generic provider module

In preperation for coc support

lua/symbols-outline/providers/init.lua

@@ -0,0 +1,19 @@
+local M = {}
+
+local providers = {
+    'symbols-outline/providers/nvim-lsp',
+    'symbols-outline/providers/markdown'
+}
+
+---@param on_symbols function
+function M.request_symbols(on_symbols)
+   for _, value in ipairs(providers) do
+       local provider = require(value)
+       if provider.should_use_provider(0) then
+           provider.request_symbols(on_symbols)
+           break
+       end
+    end
+end
+
+return M