.was icon prefix: use prefix instead of fontName (#797)

Minimum version: pending

What changed

In webfont-assistant-compatible .was JSON, the icon class prefix belongs in prefix. The optional fontName field in .was files historically stored that prefix under the wrong name. New configs written by --assistant persist prefix only; fontName in .was is deprecated and will be removed in a future major release. Loading still accepts legacy .was files that only set fontName for the prefix.

This does not change webfont()'s own fontName option (output font basename), nor .was name (font display name saved in the config).

Before

Legacy .was files (and older webfont-assistant exports) could store the icon class prefix only in fontName:

{
  "name": "MyAwesomeFont",
  "fontName": "my-icon",
  "dest": "dist/fonts",
  "files": "icons/*.svg",
  "template": "css",
  "formats": ["woff2"]
}

After

Prefer prefix; new wizard output omits deprecated .was fontName:

{
  "name": "MyAwesomeFont",
  "prefix": "my-icon",
  "dest": "dist/fonts",
  "files": "icons/*.svg",
  "template": "css",
  "formats": ["woff2"]
}

Workaround on older versions

On npm releases before built-in --assistant / --assistant-config, use webfont-assistant or hand-edit .was JSON. Either prefix or legacy fontName can carry the icon class prefix until the breaking removal ships.

After upgrading

Install the release that includes built-in assistant support, then:

1. Replace deprecated .was fontName (when it meant icon prefix) with prefix.
2. Keep name as the font display name and templateFontName when set.
3. Rerun with webfont --assistant-config path/to/config.was.