Mark `linewrapping` `open_files` table as weak

We weren't correctly garbage-collecting `Doc`s, so we had `Highlighter`s 
stay alive over their due time.

data/plugins/linewrapping.lua

@@ -310,7 +310,7 @@ local function get_line_col_from_index_and_x(docview, idx, x)
 end
 
 
-local open_files = {}
+local open_files = setmetatable({ }, { __mode = "k" })
 
 local old_doc_insert = Doc.raw_insert
 function Doc:raw_insert(line, col, text, undo_stack, time)