Widen StringScanner#[] to accept named-capture String/Symbol keys

`StringScanner#[]` accepts a capture-group name as a String or Symbol
in addition to an Integer index — the method's own RD doc in this file
documents and demonstrates it ("`specifier` symbol or string. returns
the named subgroup", with `scanner[:wday]` / `scanner['wday']`
examples). The signature only listed `Integer`, so named-capture
lookups type-check as an argument error.

Author: zonuexe

stdlib/strscan/0/string_scanner.rbs

@@ -498,7 +498,7 @@ class StringScanner
   # scanner[0]           # => nil
   # scanner[1]           # => nil
   #
-  def []: (Integer) -> String?
+  def []: (Integer | String | Symbol) -> String?
 
   # <!--
   #   rdoc-file=ext/strscan/strscan.c