change flatten to Array#flat (Node 10.x EOL)

package.json

@@ -28,7 +28,6 @@
     "create-error": "^0.3.1",
     "default-value": "^1.0.0",
     "execall": "^2.0.0",
-    "flatten": "^1.0.3",
     "indent-string": "^4.0.0",
     "is-arguments": "^1.0.4",
     "supports-color": "^7.1.0",

src/api/validate-arguments/index.js

@@ -1,7 +1,6 @@
 "use strict";
 
 const isArguments = require("is-arguments");
-const flatten = require("flatten");
 const asExpression = require("as-expression");
 const syncpipe = require("syncpipe");
 
@@ -68,7 +67,7 @@ function applyDefinitions(args, definitions, remainingArgumentsIndex) {
 	return {
 		errors: syncpipe(results, [
 			(_) => _.map((result) => result.errors),
-			(_) => flatten(_) // TODO: Switch to `Array#flat` once Node 10.x goes EOL (April 2021)
+			(_) => _.flat()
 		]),
 		newValue: results.map((result) => result.newValue)
 	};