Fix frozen string literal mutation

There's a warning

```shell
ruby_lex.rb:987: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information
```

which will lead to an error when frozen strings literals (MRI 3.5?) will be frozen by default.

Author: viralpraxis

lib/yard/parser/ruby/legacy/ruby_lex.rb

@@ -978,7 +978,7 @@ def identify_gvar
       end
 
       def identify_identifier
-        token = ""
+        token = String.new(encoding: Encoding::UTF_8)
         token.concat getc if peek(0) =~ /[$@]/
         token.concat getc if peek(0) == "@"