Meta/ShellCompletions: Add toolchain completion

This is very convenient for anyone like me who regularly runs the Clang
toolchain. The toolchain is not completed for Lagom and the
toolchain-independent help command.

Meta/ShellCompletions/zsh/_serenity

@@ -5,6 +5,7 @@ _serenity() {
     args=(
         '1:command:->commands'
         '2:target:->targets'
+        '3:toolchain:->toolchains'
         '*:: :->args'
     )
 
@@ -34,6 +35,12 @@ _serenity() {
         'lagom:Target host machine'
     )
 
+    local toolchains
+    toolchains=(
+        'GNU:Toolchain gcc or $SERENITY_TOOLCHAIN (default)'
+        'Clang:Toolchain clang'
+    )
+
     _arguments -C -S "$args[@]"
 
     local command
@@ -42,6 +49,9 @@ _serenity() {
     local target
     target="$line[2]"
 
+    local toolchain
+    toolchain="$line[3]"
+
     case "$state" in
         commands)
             _describe 'command' commands
@@ -59,6 +69,12 @@ _serenity() {
             esac
             _describe 'target' targets
             ;;
+        toolchains)
+            if [[ "$command" != help && "$target" != lagom ]]; then
+                # Toolchain-dependent invocations.
+                _describe 'toolchain' toolchains
+            fi
+            ;;
         args)
             ;;
     esac