Always print directives with double quotes when minified

[overlookmotel]

Q	A
Fixed Issues?	n/a
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	Yes
Documentation PR Link	n/a
Any Dependency Changes?	No
License	MIT

@babel/generator outputs all strings with double quotes when minified option is enabled - presumably because it gzips better with consistent quotes.

This PR makes generator do the same for directives.

generate( parse("'use strict';").program.directives[0], { minified: true } ).code

Result before: 'use strict';
Result after: "use strict";

[nicolo-ribaudo]

I slightly reworded the PR title so that it fits in a commit header (it was too long when adding (#14094)).

[overlookmotel]

Thanks both!