Add dropzonejs composer merge

[andybroomfield]

See localgovdrupal/localgov#729

What does this change?

Adds the wikimedia composer merge plugin, and then uses that to load dropzonejs composer.libraries.json to require the dropzone JS library.

How to test

Composer install or composer update --lock should install the dropzonejs libary once that module is installed.

How can we measure success?

Module gets installed

Have we considered potential risks?

Others may want to have their own dropzone library, so will have to remove this.
Those who have created localgov drupal projects already will need to copy this step.

Images

n/a

Accessibility

n/a

[finnlewis]

Discussing in Merge Tuesday:

@stephen-cox leaning towards composer-merge plugin rather than the npm asset packagist

I suggest we test this again, get the tests running, then merge this to get things moving on localgovdrupal/localgov#729

Note: we'll need to add notes to the README.md and release notes to clarify this appraoch.