main: Honor shwordsplit when expanding parameters

Closes #687, #818.

highlighters/main/main-highlighter.zsh

@@ -477,7 +477,11 @@ _zsh_highlight_main_highlighter__try_expand_parameter()
             ;;
           (*)
             # scalar, presumably
-            words=( ${(P)MATCH} )
+            if [[ $zsyh_user_options[shwordsplit] == on ]]; then
+              words=( ${(P)=MATCH} )
+            else
+              words=( ${(P)MATCH} )
+            fi
             ;;
         esac
         reply=( "${words[@]}" )

highlighters/main/test-data/opt-shwordsplit1.zsh

@@ -36,5 +36,5 @@ ed() { command ed "$@" }
 BUFFER=$'$EDITOR'
 
 expected_region_highlight=(
-  '1 7 function "issue #687"' # $EDITOR
+  '1 7 function' # $EDITOR
 )

highlighters/main/test-data/parameter-expansion-shwordsplit.zsh

@@ -0,0 +1,38 @@
+#!/usr/bin/env zsh
+# -------------------------------------------------------------------------------------------------
+# Copyright (c) 2021 zsh-syntax-highlighting contributors
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without modification, are permitted
+# provided that the following conditions are met:
+#
+#  * Redistributions of source code must retain the above copyright notice, this list of conditions
+#    and the following disclaimer.
+#  * Redistributions in binary form must reproduce the above copyright notice, this list of
+#    conditions and the following disclaimer in the documentation and/or other materials provided
+#    with the distribution.
+#  * Neither the name of the zsh-syntax-highlighting contributors nor the names of its contributors
+#    may be used to endorse or promote products derived from this software without specific prior
+#    written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR
+# IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+# FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR
+# CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER
+# IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+# OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+# -------------------------------------------------------------------------------------------------
+# -*- mode: zsh; sh-indentation: 2; indent-tabs-mode: nil; sh-basic-offset: 2; -*-
+# vim: ft=zsh sw=2 ts=2 et
+# -------------------------------------------------------------------------------------------------
+
+setopt sh_word_split
+local foo='echo foo'
+
+BUFFER='$foo'
+
+expected_region_highlight=(
+  '1 4 builtin' # $foo
+)