I18N: Add JavaScript translation support.

Adds the `wp_set_script_translations()` function which registers translations for a JavaScript file. This function takes a handle, domain and optionally a path and ensures JavaScript translation files are loaded if they exist.

Merges [43825,43828,43859,43898] from the 5.0 branch to trunk.

Props herregroen, atimmer, omarreiss, nerrad, swissspidy, ocean90, georgestephanis.
Fixes #45103, #45256.


Built from https://develop.svn.wordpress.org/trunk@44169


git-svn-id: https://core.svn.wordpress.org/trunk@43999 1a063a9b-81f0-0310-95a4-ce76da25c4cd