[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. 
I don't like this. When I execute `:!cmd` or `system()`, I expect fish to process the expression as things like `dir/**.ext` glob differently with `sh` and `bash`. Vim supports the fish shell, though, admittedly, some plugins do not.