static/completions/yak.zsh: Improve completion of commands after --.

I have found that yak's completion breaks down in the following situation:

```ShellSession
$ yak env -- <TAB>
```

I expect it to expand possible executables, but instead, it completes filenames (which is just a Zsh
fallback shining though, i believe).  A side-effect is that completions for commands i rely on also
break:

```ShellSession
$ yak env -- aws <TAB> # Completes external commands, instead of aws-subcommands
```

This improvement does two things:

* Enable the `-S` flag to `_arguments` as documented here:
  https://zsh.sourceforge.io/Doc/Release/Completion-System.html#Completion-Functions.  Specifically,
  "-S -> Do not complete options after a ‘--’ appearing on the line, and ignore the ‘--’."

* Don't modify the `words` array ourselves; just call `_normal` completion once we've got an
  environment specified in positional argument 1.  Also, use 3 instead of 2 colons for
  `'*:::command...`, which according to the docs means: With three colons before the message, the
  words special array and the CURRENT special parameter are modified to refer only to the normal
  arguments covered by this description.

* Finally, i've removed `--` as an explicit option, because `-S` takes care of this in a nice
  built-in way.

Completions now work as i expect them to, with or without the `--` between yak and the command i'm
keen to run.

static/completions/yak.zsh

@@ -2,7 +2,8 @@
 
 function _yak {
     local roles=($(yak --list-roles --cache-only 2>/dev/null))
-    _arguments '-h[Display this help message and exit]' \
+    _arguments -S \
+               '-h[Display this help message and exit]' \
                '--help[Display this help message and exit]' \
                '-l[List available AWS roles and exit]' \
                '--list-roles[List available AWS roles and exit]' \
@@ -15,12 +16,5 @@ function _yak {
                '--no-cache[Ignore cache for this request. Mutually exclusive with --cache-only]' \
                '--cache-only[Only use cache, do not make external requests. Mutually exclusive with --no-cache]' \
                '--version[Print the current version and exit]' \
-               '--[Terminator for -- flags - necessary if you would like to pass -/-- flags to subcommands]' \
-               '1:environment:(${roles})' '*::arguments: _yak_command'
-}
-
-function _yak_command {
-    shift words
-    (( CURRENT-- ))
-    _normal
+               '1:environment:(${roles})' '*:::command:_normal'
 }