refacto: deprecate used_by in parsers.lua

- remove print_warning function from utils.lua (unused)
- cleanup some functions in parsers.lua (parameters overloading
  and wrong bufnr used).
- log a deprecation notice when using used_by in a parser definition
- default the filetype_to_parsername table to the list of filetypes
  previously in the used_by keys
- update the README to indicate that change

README.md

@@ -355,12 +355,19 @@ parser_config.zimbu = {
     generate_requires_npm = false, -- if stand-alone parser without npm dependencies
     requires_generate_from_grammar = false, -- if folder contains pre-generated src/parser.c
   },
-  filetype = "zu", -- if filetype does not agrees with parser name
-  used_by = {"bar", "baz"} -- additional filetypes that use this parser
+  filetype = "zu", -- if filetype does not match the parser name
 }
 EOF
 ```
 
+If you wish to set a specific parser for a filetype, you should extend the `filetype_to_parsername` table:
+```vim
+lua <<EOF
+local ft_to_parser = require"nvim-treesitter.parser".filetype_to_parsername
+filetype_to_parsername.someft = "python" -- the someft filetype will use the python parser and queries.
+EOF
+```
+
 4. Start `nvim` and `:TSInstall zimbu`.
 
 You can also skip step 2 and use `:TSInstallFromGrammar zimbu` to install directly from a `grammar.js` in the top-level directory specified by `url`.