move method icon definition to documentation tag

[Frizi]

Changed method icon definition syntax from Icon: icon_name to ICON icon_name. That way we are consistent with the existing usage of doctags for ALIAS and eventually GROUP, and it will be easier to automate migration to method annotations when it's time to do so (see https://github.com/orgs/enso-org/discussions/7120 for related proposal).

Additionaly, I've modified the icons example scene, such that it is now possible to grab the icon names from there. This should serve as a reference for the libs team when assigning the icons.

When using run ide watch script to run the electron application, that scene can be accessed through the web browser using URL http://localhost:8080/?startup.entry=icons

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.

[jdunkerley]

Just approving the Enso bit.

[wdanilo]

👍

[farmaazon]

QA report: 🟢

Just fix the tests, as those are failing on CI.