Fix parse error of system default /usr/share/nano/json.nanorc

(Recent) `nano` packages in Ubuntu come with some `.nanorc` files preinstalled.

jline's `NanorcParser` sadly fails parsing `/usr/share/nano/json.nanorc` that comes with Ubuntu's package.

In particular, it fails at this line in `/usr/share/nano/json.nanorc`:
```
color brightblue "[][]"
```
with the exception
```
java.util.regex.PatternSyntaxException: Unclosed character class near index 3
[][]
   ^
...
	at org.jline.builtins.SyntaxHighlighter$NanorcParser.doPattern(SyntaxHighlighter.java:696)
...
```

Fixes jline#1156

builtins/src/main/java/org/jline/builtins/SyntaxHighlighter.java

@@ -693,6 +693,11 @@ private void addHighlightRule(String reference, List<String> parts, boolean case
         }
 
         private Pattern doPattern(String regex, boolean caseInsensitive) {
+            if ("[][]".equals(regex)) {
+                // Some nanorc files, for example json.nanorc, contain this regex that's valid in Nano,
+                // but not in Java. See https://github.com/jline/jline3/issues/1156
+                regex = "[\\]\\[]";
+            }
             return caseInsensitive ? Pattern.compile(regex, Pattern.CASE_INSENSITIVE) : Pattern.compile(regex);
         }
     }