Update import example component to glimmer

[geneukum]

Let's update the ImportExample component to extend glimmer/component instead of ember/component. Glimmer components offer a bunch of various advantages over classic components and are the direction that ember is heading in general.

While we're here, let's also prefer single quotes to double quotes throughout our template file.

Finally, let's make sure that we're using angle bracket syntax rather than curly brace syntax to invoke the component in our tests.

[geneukum]

This should resolve the app/components/import-example.js section of https://github.com/ember-learn/ember-api-docs/issues/805.