refactor(compiler-cli): linker - add Babel plugin, FileLinker and initial PartialLinkers (#39116)

This commit adds the basic building blocks for linking partial declarations.
In particular it provides a generic `FileLinker` class that delegates to
a set of (not yet implemented) `PartialLinker` classes.

The Babel plugin makes use of this `FileLinker` providing concrete classes
for `AstHost` and `AstFactory` that work with Babel AST. It can be created
with the following code:

```ts
const plugin = createEs2015LinkerPlugin({ /* options */ });
```

PR Close #39116

packages/compiler-cli/linker/test/file_linker/helpers.ts

@@ -0,0 +1,17 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+import * as ts from 'typescript';
+
+/**
+ * A simple helper to render a TS Node as a string.
+ */
+export function generate(node: ts.Node): string {
+  const printer = ts.createPrinter({newLine: ts.NewLineKind.LineFeed});
+  const sf = ts.createSourceFile('test.ts', '', ts.ScriptTarget.ES2015, true);
+  return printer.printNode(ts.EmitHint.Unspecified, node, sf);
+}