FEAT(`show-guidelines`): provide numeric values for overlay guidelines

We have [show-guidelines](https://flutter.dev/docs/development/tools/devtools/inspector#show-guidelines), which is a superb feature on its own.

### Improvement (what?)
However on top of this feature. What could be even better is the ability to toggle display of numeric values of the overlay lines themselves. This numeric value would show within the flutter app overlay and not editor.

### Motivation (why?)
This helps a lot as it guides the **app developer** to understand if the computed runtime constraints/width/height/spacers do actually match the UI file provided by the **app designer**.

As most of the times, there is a designer who provides UI for the developer to follow via apps/services such as AdobeXD, Figma, Zeplin, InVision, Adobe Illustrator, and the list goes on. (not affiliated with any)

### Developer-Facing Interaction (how?)
This toggle could happen:
- A. Within the widget inspector controls, similar to how `show-guidelines` feature is toggled. This shows all numeric values for all lines on the screen. This is the **verbose** way to do it.
- B. Because option A. may be too verbose and pollute the UI with lots of numbers, another mechanism of showing numeric values is a simple hover. By hovering over the respective line on the app overlay, widget inspector can show numeric value of the length of the line being hovered over, while hiding numeric values of other lines. Of course this option is not mutually exclusive with option A and both can be useful to implement.

Start by reviewing the linked show-guidelines documentation and the widget inspector controls that toggle it. Determine whether the desired scope is the verbose numeric overlay, hover-based values, or both; the work is done when the selected interaction displays guideline measurements inside the Flutter app overlay without relying on the editor.

索引モデルが issue の本文から書いたものです。