refactor: Add a generic provider module

In preperation for coc support

lua/symbols-outline/providers/markdown.lua

@@ -0,0 +1,15 @@
+local md_parser = require('symbols-outline.markdown')
+
+local M = {}
+
+-- probably change this
+function M.should_use_provider(bufnr)
+    return vim.api.nvim_buf_get_option(bufnr, 'ft') == 'markdown'
+end
+
+---@param on_symbols function
+function M.request_symbols(on_symbols)
+    on_symbols(md_parser.handle_markdown())
+end
+
+return M