This adds the following Vim commands: 
- `:r[ead] [name]`
- `:{range}r[ead] [name]`

The most important parts of this feature are outlined
[here](https://vimhelp.org/insert.txt.html#%3Ar).

The only intentional difference between this and Vim is that Vim only
allows `:read` (no filename) for buffers with a file attached. I am
allowing it for all buffers because I think that could be useful.

Release Notes:

- vim: Added the [`:r[ead] [name]` Vim
command](https://vimhelp.org/insert.txt.html#:read)

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>