* Add possibility to declare optional parameters and switch flags
With this commit applied it is now possible to specify optional parameters and flags
as switches. This PR **only** makes guarantees about **parsing** optional flags and
switches correctly. This PR **does not guarantee flawless functionality** of
optional parameters / switches within scripts.
functionality within scripts. Example:
test.nu
```shell
def my_command [
opt_param?
opt_param2?: int
--switch
] {echo hi nushell}
```
```shell
> source test.nu
> my_command -h
───┬─────────
0 │ hi
1 │ nushell
───┴─────────
Usage:
> my_command <mandatory_param> (opt_param) (opt_param2) {flags}
Parameters:
<mandatory_param>
(opt_param)
(opt_param2)
Flags:
-h, --help: Display this help message
--switch
--opt_flag <any>
```
* Update def docs
