[WIP] Set some defaults early

Implements idea from issue #1664 

- [x] Create `runtime/defaults.vim` (definition of contents ongoing)
- [x] Load `runtime/defaults.vim` from `main.c:source_startup_scripts()`.
- [x] Update `runtime/doc/starting.txt`.

Notes: `defaults.vim` is mostly based on vim-sensible. I removed feature checks since they are unneded in neovim. 
Yes, as a fish user I am not too keen on this either. But some things are weird with the current default, mostly because fish will always spit out control codes (see `!ls`).