main: Highlight backticks in double quotes

highlighters/main/main-highlighter.zsh

@@ -894,7 +894,7 @@ _zsh_highlight_main_highlighter_highlight_single_quote()
 # Highlight special chars inside double-quoted strings
 _zsh_highlight_main_highlighter_highlight_double_quote()
 {
-  local -a match mbegin mend
+  local -a match mbegin mend saved_reply
   local MATCH; integer MBEGIN MEND
   local i j k style
   reply=()
@@ -904,6 +904,12 @@ _zsh_highlight_main_highlighter_highlight_double_quote()
     (( k = j + 1 ))
     case "$arg[$i]" in
       '"') break;;
+      '`') saved_reply=($reply)
+           _zsh_highlight_main_highlighter_highlight_backtick $i
+           (( i = REPLY ))
+           reply=($saved_reply $reply)
+           continue
+           ;;
       '$' ) style=dollar-double-quoted-argument
             # Look for an alphanumeric parameter name.
             if [[ ${arg:$i} =~ ^([A-Za-z_][A-Za-z0-9_]*|[0-9]+) ]] ; then

highlighters/main/test-data/back-quoted-argument.zsh

@@ -28,9 +28,12 @@
 # -------------------------------------------------------------------------------------------------
 
 # 42 is in the command position in a nested subshell.
-BUFFER='echo `echo \`42\`` `echo 6 times 9'
+BUFFER='echo `echo \`42\`` "is `echo equal` to" `echo 6 times 9'
 
 expected_region_highlight=(
-  "6 18 back-quoted-argument"
-  "20 34 back-quoted-argument-unclosed"
+  "6 18 back-quoted-argument" # `echo \`42\``
+  "20 23 double-quoted-argument" # "is
+  "24 35 back-quoted-argument" # `echo equal`
+  "36 39 double-quoted-argument" # to"
+  "41 55 back-quoted-argument-unclosed" # `echo 6 times 9
 )