Emit style modules via importmap + dataURI instead of <style type="module">

[KurtCattiSchmidt]

<style type="module"> is going to need some spec updates and will probably have some syntax changes before we can ship it. In the meantime, we can simulate it via an Import Map and a dataURI. Performance characteristics are very similar between the two, with small stylesheets and duplicated stylesheets actually being slightly faster than <style type="module">.

This change updates the Rust BTR code with the "module" strategy to output an equivalent Import Map instead of <style type="module">. The serde_json Rust library is used for JSON handling, which is used elsewhere in WebUI, so this is not a new dependency.

e.g. this:

<style type="module" specifier="foo">
  span {color: blue;}
</style>

...would be replaced by:

<script type="importmap">
{
  "imports": {
    "foo": "data:text/css,span {color: blue;}"
  }
}
</script>

...where the CSS content is URL-encoded in the Import Map version. This depends on Multiple Import Map support, which was enabled in Chromium 133. Mozilla and WebKit have positive support for Multiple Import Maps but haven't shipped it yet. Since the "module" strategy also depends on shadowrootadoptedstylesheets support (which hasn't shipped yet), it should be fine to add this new dependency.