Improve support for multi-word completion strings in ZSH (#216)

* Improve support for multi-word completions in Z shell

I will not pretend to fully understand why this new "expand string into array" expression works when the other didn't, but it does. The new expression is based on this SO answer: https://unix.stackexchange.com/a/29748.

The motivation for this change was for the install command of xcodes (https://github.com/RobotsAndPencils/xcodes) to support Xcode version completion strings with multiple words, like "11.6 Beta". The previous version of this expression would split this string into two, so that "11.6" and "Beta" were independent options in the ZSH completion UI, which didn't make sense for this use case.

Author: interstateone

Sources/ArgumentParser/Completions/ZshCompletionsGenerator.swift

@@ -22,7 +22,7 @@ struct ZshCompletionsGenerator {
 
     \(generateCompletionFunction([type]))
     _custom_completion() {
-        local completions=($($*))
+        local completions=("${(@f)$($*)}")
         _describe '' completions
     }
 

Tests/ArgumentParserExampleTests/MathExampleTests.swift

@@ -495,7 +495,7 @@ _math_help() {
 
 
 _custom_completion() {
-    local completions=($($*))
+    local completions=("${(@f)$($*)}")
     _describe '' completions
 }
 

Tests/ArgumentParserUnitTests/CompletionScriptTests.swift

@@ -154,7 +154,7 @@ _base() {
 
 
 _custom_completion() {
-    local completions=($($*))
+    local completions=("${(@f)$($*)}")
     _describe '' completions
 }
 
@@ -226,7 +226,7 @@ _escaped() {
 
 
 _custom_completion() {
-    local completions=($($*))
+    local completions=("${(@f)$($*)}")
     _describe '' completions
 }