main: Refresh highlighting upon accept-* widgets (accept-line et al).

Use this to remove [path_prefix] highlighting during accept-* widgets.
This causes ": file.tx<CR>" to remove the underline from "file.tx", like
": file.tx<SPACE>" already does.  (Assume 'file.txt' exists.)

highlighters/main/test-data/path_prefix2.zsh

@@ -35,5 +35,5 @@ BUFFER='ls /bin/s'
 WIDGET=accept-line
 
 expected_region_highlight=(
-  "4 9 $ZSH_HIGHLIGHT_STYLES[default] 'implemented by next commit'"    # /bin/s
+  "4 9 $ZSH_HIGHLIGHT_STYLES[default]"    # /bin/s
 )