Support passing HTML/lit templates as arguments to l10n strings

[LeoMcA]

There are times when we need to pass HTML/lit templates as an argument to a fluent string, something like:

baseline-indicator-alternatives-consider = Consider using the following features instead: { $list }.

const links = alternatives.map(
  ({ name, description, mdn_url }) =>
    html`<a
      href=${changeDocsLocale(mdn_url, context.locale)}
      title=${description}
      >${name}</a
    >`,
);
const parts = new Intl.ListFormat(context.locale, {
  type: "disjunction",
}).formatToParts(links.map((_, i) => String(i)));
const list = parts.map(({ type, value }) =>
  type === "element" ? links[Number(value)] : value,
);
context.l10n.raw({ id: "baseline-indicator-alternatives-consider", args: { list }})

Currently this fails with:

  TypeError: Variable type not supported: $list, object
      at resolveVariableReference (file:///home/leo/Projects/mdn/fred/node_modules/@fluent/bundle/esm/resolver.js:137:1)
      at resolveExpression (file:///home/leo/Projects/mdn/fred/node_modules/@fluent/bundle/esm/resolver.js:86:1)
      at resolveComplexPattern (file:///home/leo/Projects/mdn/fred/node_modules/@fluent/bundle/esm/resolver.js:254:1)
      at FluentBundle.formatPattern (file:///home/leo/Projects/mdn/fred/node_modules/@fluent/bundle/esm/bundle.js:142:46)
      at Fluent.getMessage (file:///home/leo/Projects/mdn/fred/l10n/fluent.js:169:1)
      at Fluent.get (file:///home/leo/Projects/mdn/fred/l10n/fluent.js:65:1)
      at l10n.raw (file:///home/leo/Projects/mdn/fred/l10n/fluent.js:283:1)
      at BaselineIndicator.normalizeData (file:///home/leo/Projects/mdn/fred/components/baseline-indicator/server.js:170:1)
      at BaselineIndicator.render (file:///home/leo/Projects/mdn/fred/components/baseline-indicator/server.js:210:1)
      at BaselineIndicator.render (file:///home/leo/Projects/mdn/fred/components/server/index.js:49:1)

We could/should probably also come up with a helper for the dance we have to do with substituting back into the formatToParts list (it requires strings to be passed).