feat: Change newline_separated support multiple newlines

[joesun-google]

1. Change the option newline_separated to a new IntOrBool type that treat no=0 and yes=1,
2. Change the implementation to check for and add N newlines, when newline_separated=N is specified,
3. Add new test cases.

Tested :

$ go test ./...
?       github.com/google/keep-sorted   [no test files]
?       github.com/google/keep-sorted/cmd       [no test files]
ok      github.com/google/keep-sorted/goldens   (cached)
ok      github.com/google/keep-sorted/keepsorted        (cached)