Add support for modifying import map as a js variable

[joeldenning]

Since only one import map per page is supported in the most recent import maps spec, we should provide a way for import map overrides to work with only one import map.

See more info at https://github.com/WICG/import-maps/#dynamic-import-map-example

<script>
  window.importMap = {imports: {foo: './foo1.js'}};
<script>
<script src="https://cdn.jsdelivr.net/npm/import-map-overrides/dist/import-map-overrides.js"></script>

With this proposed API, import map overrides would look for a global variable named importMap, modify it with the overrides, and then inject it as <script type="importmap"> into the DOM.

[joeldenning]

Oo actually a better API would be this, I think:

<script type="overridable-importmap">
{
  "imports": {
    "foo": "./foo1.js"
  }
}
</script>
<script src="https://cdn.jsdelivr.net/npm/import-map-overrides/dist/import-map-overrides.js"></script>