This is cherry-picked from these Vim patches:

Only applicable change outside vi_diff.txt in patch 8.1.1226:
https://github.com/vim/vim/commit/6c60f47fb9251e686217d51cf81847e14d0dd26d

Most changes outside starting.txt and vi_diff.txt in patch 8.1.1280:
https://github.com/vim/vim/commit/25c9c680ec4dfbb51f4ef21c3460a48d3c67ffc8

Missing docs for 'mousemoveevent':
https://github.com/vim/vim/commit/cbaff5e06ec525d31dc44093125c42029e01d508

Allow Include What You Use to remove unnecessary includes and only
include what is necessary. This helps with reducing compilation times
and makes it easier to visualise which dependencies are actually
required.

Work on https://github.com/neovim/neovim/issues/549, but doesn't close
it since this only works fully for .c files and not headers.

Adds a `name` key to the opts dict passed to Lua command callbacks
created using `nvim_create_user_command()`. This is useful for when
multiple commands use the same callback.

Note that this kind of behavior is not as strange as one might think,
even some internal Neovim commands reuse the same internal C function,
differing their behavior by checking the command name. `substitute`,
`smagic` and `snomagic` are examples of that.

This will also be useful for generalized Lua command preview functions
that can preview a wide range of commands, in which case knowing the
command name is necessary for the preview function to actually be able
to execute the command that it's supposed to preview.

fix(api)!: nvim_parse_cmd omit "count" "range" "reg" if not supported

Allows `nvim_cmd` to use the first argument as count for applicable
commands. Also adds support for non-String arguments to `nvim_cmd`.

I don't think using an integer as a NUL-terminated string can work on
big-endian systems, at least.

This is also not tested. Add a test.

Also fix a mistake in the docs of nvim_parse_cmd.