add Completion class

src/console/completion.js

@@ -0,0 +1,26 @@
+export default class Completion {
+  constructor(completions) {
+    if (typeof completions.length !== 'number') {
+      throw new TypeError('completions does not have a length in number');
+    }
+    this.completions = completions
+    this.index = 0;
+  }
+
+  prev() {
+    if (this.completions.length === 0) {
+      return null;
+    }
+    this.index = (this.index + this.completions.length - 1) % this.completions.length
+    return this.completions[this.index];
+  }
+
+  next() {
+    if (this.completions.length === 0) {
+      return null;
+    }
+    let item = this.completions[this.index];
+    this.index = (this.index + 1) % this.completions.length
+    return item;
+  }
+}