runtime(sh): Update syntax, highlight escaped chars in test expressions

Highlight escape characters in unquoted test expression operands.

E.g., [[ foo == \[bar\] ]]

fixes #17221

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/sh_14.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+# Issue #17221 (sh syntax: escaped square brackets don't work in [[ ]])
+
+[[ foo == [bar] ]]
+[[ foo == \[bar\] ]]
+
+echo [foo]
+echo \[foo\]