[Reader Mode] Add analysis and visualization tools to the Test Extension.

This CL adds to the Test Extension the following features for analysis
and debugging on the cloned page: Metadata extraction, DOM boxifying,
and a Readability analysis visualizer.

These features provide deeper insight into the distillation process:
* **Extract Metadata**: Displays the raw data parsed by Readability.js
  (e.g., title, byline, excerpt) in a tabular format, for quick
  inspection.
* **Boxify DOM**: Draws an outline around every single element on the
  page. This helps to understand DOM structure by identifying Elements
  large and small, and finding invisible elements that might influence
  layout.
* **Visualize Readability**: Highlights the results of the Readability
  algorithm on the page. Kept elements are green; discarded ones red.
  This provides immediate visual feedback on the algorithm's decisions.

To support an iterative developer workflow (e.g., modifying the DOM via
DevTools and re-running a tool), the visualization features are
idempotent: They clean up any artifacts from their own previous runs
before applying new effects.

Additional refactoring:
* Standardize the messaging protocol (using a `target` field). This was
  necessary to fix a bug where actions on the original page could
  incorrectly trigger listeners on the cloned page.
* Make function naming consistent with commands they handle (e.g.,
  `run[Command]`).
* On the cloned page: Adopt a synchronous, eager-loading script
  strategy. This simplifies the code and improves the debugging workflow
  by allowing developers to set breakpoints in symlinked files (e.g.,
  Readability.js) on page load.

Details:
* Add `metadata.html`, `metadata.js`, `metadata.css`: New pages and
  scripts to display extracted article metadata in a table.
* Add `metadata_processor.js`, `metadata_extractor.js`: Shared modules
  to invoke Readability.js and to extract only the metadata fields.
* Update `cloned.js` to implement client-side logic for the Visualize
  and Boxify features.
* Update `popup.js` to add the "Metadata", "Boxify", and "Visualize"
  buttons to the UI for the cloned page context.
* Update `background.js` to add handlers for the new commands.
* Update `manifest.json` to make all new metadata-related files web
  accessible.
* Update `README.md` to document all the new features and their usage.

Bug: 449799192
Change-Id: Iaed273db770d665782cbe5a3376ff06f50644922
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7042192
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Brandon Wylie <wylieb@google.com>
Cr-Commit-Position: refs/heads/main@{#1530382}