I2I Move Bento code into a new `src/bento` directory

### Summary

Move all the Bento extensions and preact code into a new `/bento` directory.

### Design Document

[The Future of Bento Development](https://docs.google.com/document/d/1ZIOSsDleyKyyYtZ-W1XS_eYSuDs30sRWELyVtsg2rN0/edit#heading=h.xgjl2srtytjt)

### Motivation

Bento source code being stored in the `/extensions` directory alongside their AMP wrappers is not obvious to new developers and obscures the boundary between Bento and AMP. Moving the code to a new directory will provide us with the opportunity to clearly define the boundary and eventually make it easier to move Bento to a new repository.

### Alternative Solutions

Bento could also be moved to `/src/bento`

### Launch Tracker

_No response_

### Notifications

/cc @ampproject/wg-approvers

Start by reading the linked “The Future of Bento Development” design document and inventory the Bento extensions and Preact code currently stored under /extensions. Compare the proposed /bento and /src/bento layouts, then verify that all Bento source has moved while its boundary from AMP wrappers remains clear.

Written by the indexing model from the issue text.