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/README.md

@@ -0,0 +1,18 @@
+# Angular Linker
+
+This package contains a `FileLinker` and supporting code to be able to "link" partial declarations of components, directives, etc in libraries to produce the full definitions.
+
+The partial declaration format allows library packages to be published to npm without exposing the underlying Ivy instructions.
+
+The tooling here allows application build tools (e.g. CLI) to produce fully compiled components, directives, etc at the point when the application is bundled.
+These linked files can be cached outside `node_modules` so it does not suffer from problems of mutating packages in `node_modules`.
+
+Generally this tooling will be wrapped in a transpiler specific plugin, such as the provided [Babel plugin](./babel).
+
+## Unit Testing
+
+The unit tests are built and run using Bazel:
+
+```bash
+yarn bazel test //packages/compiler-cli/linker/test
+```