This preserves backwards compatibility still, but that will be removed at
some point when dropping .6. At that point, another refactor can
simplify many of the functions used.
Now building a list of filetypes that already have language servers
configured and then excluding those filetypes from null-ls.
There is an override where they can be forced enabled for language
servers that may not have a formatter or where the null-ls formatter is
preferred. Eg. Python where I want to format with black.
