Fix embroider compatibility

[ef4]

This package defines an empty module for the convenience of typescript users, so they can say import "qunit-dom" to get the types loaded. It's not really needed for any actual code, because the code gets applied directly to QUnit's own objects.

But because this package declares itself an ember-addon, its own module namespace is expected to be defined by real modules in treeForAddon or treeForAddonTestSupport. Not a dynamic define in vendor.

define() is most commonly used for things that aren't in an addon's own module namespace, and that continues to work under embroider. But this more unusual case -- an addon define()ing itself -- is not supported.

Without this change, attempting to import "qunit-dom" fails under embroider.

[ef4]

I changed this so the dummy module is expressed as named AMD. This requires me to be more nuanced about what embroider supports, since I just said above that an addon can't define() its own modules, and it looks like my PR is doing that.

To be more precise, an addon can't define() its own modules from other arbitrary files (like vendor/define-dummy-module.js). But we do support the case where we're handed a treeForAddon or treeForAddonTestSupport which happens to have been precompiled to named AMD. This case works because the files are all still in their correctly resolvable locations, and the names that are being defined() precisely match the real filesystem layout.