Pass Babel AST directly

**I'm submitting a feature request**

**Webpack Version:**

`v4.0.0 (@next)`

**Babel Core Version**:

`v7.0.0 (@next)`

**Babel Loader Version**:

`v8.0.0`

**Current behavior:**

Pass `result.code` `{String}`

**Desired behavior:**

Pass `result.ast` as metadata `cb(null, code, map, { webpackAST: ast /* result.ast */ })` `{Object}`

> ⚠️ Needs to be compatible to the Acorn AST (ESTree) ([webpack #5925](https://github.com/webpack/webpack/pull/5925))

* **What is the motivation / use case for changing the behavior?**

Avoid unnecessary parsing, if possible

No source file, test, or entry point is named in the issue. Start by reviewing how the loader currently passes Babel's result to webpack, then check the Acorn AST compatibility requirement in webpack #5925. Done means exposing result.ast as webpackAST metadata without the unnecessary parsing described in the request.

Written by the indexing model from the issue text.