nvim: Add rendermarkdown todo completion source

Could be optionally set for 'blink' but since blink.cmp also displays
lsp results this is more general (and, currently, the recommended way).

Suggests task states if doing a single '-' dash at an appropriate place.

nvim/.config/nvim/lua/plugins/prose.lua

@@ -106,6 +106,9 @@ local prose_plugs = {
 					conceal = false,
 				},
 			},
+			completions = {
+				lsp = { enabled = true },
+			},
 		},
 		dependencies = {
 			"nvim-treesitter/nvim-treesitter",