Empty use declarations now delete exported elements

[DuncanUszkay1]

Issue: #1296

With this PR, you can now specify an empty use statement to prevent an implementation from being injected with the following syntax: --use Date=. When attempting to use date, which is normally globally exported automatically, you will then instead get this error message:

ERROR TS2304: Cannot find name 'Date'.

 Date.now();
 ~~~~
 in main.ts(1,1)

ERROR: 1 compile error(s)

This is useful when you cannot support the module in question, which in the case of date would be due to the fact that the host will not import it.

[dcodeIO]

Let's add a simple test for this to make sure that unrelated changes don't accidentally break the behavior. Suggesting to use the Date object as a test dummy.

[dcodeIO]

Great, thanks!