--module option?

[guybedford]

Perhaps it would make sense to properly distinguish between --module and --script modes?

When using --module uglify-es could:

• Assume "use strict" by default
• Do --toplevel mangling by default as we know local variables are not globals

Ideally import/export when not using --module should throw to ensure a correct source goal interpretation, but I understand if this strictness is not desired.

[Rich-Harris]

Perhaps --module could be implicit if an import/export declaration is encountered, rather than throwing an error?

[avdg]

I believe modules and class code are always strict mode anyway (unless there is something changed with the standard, I think generator functions are non-strict by default but I'm not sure at all)