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
Pete Bacon Darwin