* Add rest arg to def
This commit applied adds the ability to define the rest parameter of a def
command. It does not implement the functionality to expand the rest argument in
a user defined def function.
The rest argument has to be exactly worded "...rest".
Example after this PR is applied:
file test.nu
```shell
def my_command [
...rest:int # My rest arg
] {
echo 1 2 3
}
```
```shell
> source test.nu
> my_command -h
Usage:
> my_command ...args {flags}
Parameters:
...args: My rest arg
Flags:
-h, --help: Display this help message
```
* Fix space in help on wrong side
We split off the evaluation engine part of nu-cli into its own crate. This helps improve build times for nu-cli by 17% in my tests. It also helps us see a bit better what's the core engine portion vs the part specific to the interactive CLI piece.
There's more than can be done here, but I think it's a good start in the right direction.
