What problem does this feature solve?

there is currently no option for SVG favicons

SVG favicons have many benefits such a small file size and support for CSS media queries such as prefers-color-scheme

What does the proposed API look like?

pwa: {
    iconPaths: {
      faviconSVG: 'img/icons/favicon.svg',
      favicon32: 'img/icons/favicon-32x32.png',
      favicon16: 'img/icons/favicon-16x16.png',
    }
  }

wich would generate

<link rel="icon" type="image/svg xml" href="/img/icons/favicon.svg">
<link rel="icon" type="image/png" sizes="32x32" href="/img/icons/favicon-32x32.png">
<link rel="icon" type="image/png" sizes="16x16" href="/img/icons/favicon-16x16.png">