Add readable, writable and duplex

snippets/isWritableStream.md

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+### isWritableStream
+
+Checks if the given argument is a writable stream.
+
+Check if the value is different from `null`, use `typeof` to check if the value is of type `object` and the `pipe` property is of type `function`.
+Additionally check if the `typeof` the `_write` and `_writableState` properties are `function` and `object` respectively.
+
+```js
+const isWritableStream = val =>
+  val !== null &&
+  typeof val === 'object' &&
+  typeof val.pipe === 'function' &&
+  typeof val._write === 'function' &&
+  typeof val._writableState === 'object';
+```
+
+```js
+const fs = require('fs');
+isWritableStream(fs.createWriteStream('test.txt')); // true
+```