add filterFalsy snippet, tags, and tests

snippets/filterFalsy.md

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+### filterFalsy
+
+Filters out the falsy values in an array.
+
+Use `Array.prototype.filter()` to get an array containing only truthy values.
+
+```js
+const filterFalsy = arr => arr.filter(Boolean)
+```
+
+```js
+filterFalsy(['', true, {}, false, 'sample', 1, 0]); // [true, {}, 'sample', 1]
+```