Migrated, following TeeJ_Dv's advice, the auto-command creation to a Lua
function. As this auto-command only cares for the on_attach Lua scope,
it seems correct to have this one defined as a Lua function instead of
the legacy Vimscript.
An argument to a Neovim's Lua function that used to live in global
scope, now needs to be declared as part of the function definition, and
passed accordingly.
Renamed Vim-script and Lua modules to separate the functions from, which
settings correspond to the client part and which to the server part of
the LSP protocol for either editor.