feat: add support for icon contribution by extensions

[benoitf]

What does this PR do?

add support for icon contribution by extensions

Screenshot/screencast of this PR

What issues does this PR fix or reference?

fixes #1525

How to test this PR?

Add contribution in an extension

something like

download and store it at the root path of an extension
https://github.com/twbs/icons/blob/main/font/fonts/bootstrap-icons.woff2

add in package.json

    "icons": {
      "my-custom-icon": {
        "description": "My icon",
        "default": {
          "fontPath": "bootstrap-icons.woff2",
          "fontCharacter": "\\F62B"
        }
      }
    }

to know the fontCharacter, use for example https://fontdrop.info (and upload the .woff2 file or create a file from svg using https://github.com/itgalaxy/webfont cli

and then for example for a status bar item, for the status icon, use instead of fa fa-something ${my-custom-icon}

unit tests provided

[deboer-tim]

I spent a long time trying to figure out why this wasn't working on my machine: the icons were being picked up and registered correctly, the StatusBarItem was rendering right, but there was no icon or font showing up in IconStyle.

I eventually figured out there was no extensions-started event - because the podman extension never finished starting on my clone. Deleting the extension made everything else fail really badly, but my icon showed up amid the mess. 👍🏼

So +1, and I'll file another bug for extension startup.