Add IsWriterTerminal and IsWriterCygwinTerminal functions (fixes #80)

## Summary Add support for `io.Writer` in addition to `io.Reader` (as suggested in https://github.com/mattn/go-isatty/pull/81#issuecomment-1469602717). Also fixes filename typo: `writter.go` -> `writer.go` (per reviewer feedback in original PR). ## Changes - `writer.go`: New file with `IsWriterTerminal` and `IsWriterCygwinTerminal` functions - `writer_test.go`: Corresponding tests ## References - Fixes #80 - Supersedes original PR #81 (cardil:feature/check-writer) which had filename typo