feat: break input types into separate classes (#7019)

* chore: move input and input types into new directory

* feat: define and export new input types

* feat: modify blocks to construct individual inputs

* chore: transition code to use actual type checks

* chore: fixup input type type

* chore: format

* chore: fixup PR comments

* chore: fix build

core/inputs/dummy_input.ts

@@ -0,0 +1,24 @@
+/**
+ * @license
+ * Copyright 2023 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type {Block} from '../block.js';
+import {Input} from './input.js';
+import {inputTypes} from './input_types.js';
+
+
+/** Represents an input on a block with no connection. */
+export class DummyInput extends Input {
+  readonly type = inputTypes.DUMMY;
+
+  /**
+   * @param name Language-neutral identifier which may used to find this input
+   *     again.
+   * @param block The block containing this input.
+   */
+  constructor(public name: string, block: Block) {
+    super(name, block, null);
+  }
+}