[DX] Allow JS in theme .info files to optionally specify header/footer as scope (default to header for BC)

Description of the need

When adding a JS file to your theme via the .info file, it's automatically added to the header (i.e. between the <head> tags of the HTML). If you instead want to add it to the footer (i.e. just before the closing <body> tag in the HTML) you need to instead add the JS via backdrop_add_js() in something like THEME_preprocess_page(), which allows you to specify 'scope' => 'footer'.

Proposed solution

It'd be nice if we could instead do something like this in the .info file:

scripts[footer][] = js/my_footer.js

That'd be much simpler and, IMO, the more expected solution.

Alternatives that have been considered

The current solution works well enough, it's just not as easy/obvious.

Additional information

Here's a question in the forum of someone expecting this functionality but finding it didn't work: https://forum.backdropcms.org/forum/how-add-custom-js-template-head-or-f...

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now allows themes to add javascript files to the footer directly from the .info file.