Add function for retrieving metadata from registered script

[desrosj]

There is currently a function for adding metadata to a registered script (wp_script_add_data()), but no function for retrieving that metadata.

My use case
I am trying to write a unit test for a function in my plugin that generates data and calls wp_localize_script() to ensure the expected data is passed for localization.

Other Use Cases
I could see other use cases where a plugin could change a metadata value or add a new key if a certain value is assigned to a certain key.

[desrosj]

Adds wp_script_get_data() with unit tests.

[desrosj]

Refreshed patch.