Proof of concept: support directives.

[cristianoc]

Allow to specify directives emitted verbatim at the top of generated js files.

See #5840

[cristianoc]

Should directives be emitted before or after 'use strict', if any.
Should one support command-line too?

[zth]

Should directives be emitted before or after 'use strict', if any. Should one support command-line too?

I think it's good if directives are emitted on top of 'use strict', if it's also being emitted. Command line?

[zth]

I changed the tests to output 'use client';, since that's the main thing this is for.