runtime(sh): Update syntax, fix single-quoted strings in parameter expansions

Ignore single-quoted backslash escape sequences in parameter expansions.

\' is not an escaped single quote in ${foo:-'word\'}.

closes: #17261

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/syntax/sh.vim b/runtime/syntax/sh.vim
index f0a6685..4add792 100644
--- a/runtime/syntax/sh.vim
+++ b/runtime/syntax/sh.vim
@@ -11,6 +11,7 @@
 "		2025 Apr 03 command substitution opening paren at EOL (#17026)
 "		2025 Apr 10 improve shell detection (#17084)
 "		2025 Apr 29 match escaped chars in test operands (#17221)
+"		2025 May 06 improve single-quote string matching in parameter expansions
 " Version:		208
 " Former URL:		http://www.drchip.org/astronaut/vim/index.html#SYNTAX_SH
 " For options and settings, please use:      :help ft-sh-syntax
@@ -649,7 +650,7 @@
  syn match  shDerefOp	contained	"[,^]\{1,2}"	nextgroup=@shDerefPatternList
  syn match  shDerefOp	contained	"@[uULQEPAKa]"
 endif
-syn region shDerefString	contained	matchgroup=shDerefDelim start=+\%(\\\)\@<!'+ end=+'+	contains=shStringSpecial
+syn region shDerefString	contained	matchgroup=shDerefDelim start=+\%(\\\)\@<!'+ end=+'+
 syn region shDerefString	contained	matchgroup=shDerefDelim start=+\%(\\\)\@<!"+ skip=+\\"+ end=+"+	contains=@shDblQuoteList,shStringSpecial
 syn match  shDerefString	contained	"\\["']"	nextgroup=shDerefPattern