feat: add custom icon - Githubissues

This PR allows to customize the icon of a step.

The icon can be:

an url: "icon": "https://cdn.jsdelivr.net/gh/vsls-contrib/code-tour/images/icon-small.png"
a relative file: "icon": "./icon.png" (relative to the workspace)
a tuple containing the icon id and, optionally, the color id: "icon": "error,terminal.ansiCyan" from
- https://code.visualstudio.com/api/references/icons-in-labels#icon-listing
- https://code.visualstudio.com/api/references/theme-color

It should fix #236.

microsoft / codetour