feat(nvim): add function to write and source files

Copied from tjdevries' config_manager repository, add a global
`save_and_exec` function that will write and source vim or lua files,
and use this within my existing `<leader>so` key mapping.

roles/neovim/files/autoload/opdavies.vim

@@ -0,0 +1,12 @@
+if !exists('*opdavies#save_and_exec')
+  function! opdavies#save_and_exec() abort
+    if &filetype == 'vim'
+      :silent! write
+      :source %
+    elseif &filetype == 'lua'
+      :silent! write
+      :luafile %
+    endif
+    return
+  endfunction
+endif