Add new "String#chars" rule

Follow rubocop/rubocop#9615.

This PR adds "String#chars" rule.

```ruby
# bad
string.split(//)
string.split('')

# good
string.chars
```

README.adoc

@@ -4487,6 +4487,20 @@ url.sub('http://', 'https://')
 str.tr('-', '_')
 ----
 
+=== `String#chars` [[string-chars]]
+
+Prefer the use of `String#chars` over `String#split` with empty string or regexp literal argument.
+
+[source,ruby]
+----
+# bad
+string.split(//)
+string.split('')
+
+# good
+string.chars
+----
+
 === `sprintf` [[sprintf]]
 
 Prefer the use of `sprintf` and its alias `format` over the fairly cryptic `String#%` method.