Add support for dynamic import operator to JavaScript mode

[tmcw]

Adds support for dynamic import syntax to the JavaScript mode. Currently this isn't handled, so, for instance, property access from dynamic imports - like

import('foo').then(() => {});

Highlights then as a variable, not a property.

This PR allows for dynamic imports at the top level by detecting (, and handles imports in expressions by just continuing to the expression mode when they're detected.

[adrianheine]

dynamic import is stage 3, but I think that's okay for CodeMirror. Could you add a test?

[tmcw]

👍 Test cases added.

[marijnh]

Thanks!