back-dollar-quoted-argument: Highlight incomplete escape sequences as unknown-token

Followup to 219184f046d40cc723606797666dab18f8a6aea7, which fixed issue #196.

highlighters/main/main-highlighter.zsh

@@ -332,6 +332,10 @@ _zsh_highlight_main_highlighter_highlight_dollar_string()
               (( k += $#MATCH ))
               (( i += $#MATCH ))
             else
+              if (( $#arg > $i+1 )) && [[ $arg[$i+1] == [xXuU] ]]; then
+                # \x not followed by hex digits is probably an error
+                style=$ZSH_HIGHLIGHT_STYLES[unknown-token]
+              fi
               (( k += 1 )) # Color following char too.
               (( i += 1 )) # Skip parsing the escaped char.
             fi

highlighters/main/test-data/dollar-quoted2.zsh

@@ -29,12 +29,14 @@
 # -------------------------------------------------------------------------------------------------
 
 ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]=$unused_highlight
-BUFFER=": \$'foo\xbar\udeadbeef'"
+BUFFER=": \$'foo\xbar\udeadbeef\uzzzz'"
 
 expected_region_highlight=(
   "3 7 $ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]" # $'foo
   "8 11 $ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]" # \xba
   "12 12 $ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]" # r
   "13 18 $ZSH_HIGHLIGHT_STYLES[back-dollar-quoted-argument]" # \dead
-  "19 23 $ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]" # beef'
+  "19 22 $ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]" # beef
+  "23 24 $ZSH_HIGHLIGHT_STYLES[unknown-token]" # \u
+  "25 29 $ZSH_HIGHLIGHT_STYLES[dollar-quoted-argument]" # zzzz'
 )