js: Declare 'use strict' on all scripts and no modules.

And enable the import/unambiguous ESLint rule as a check on our
partition between scripts and modules.  After this commit, if you add
a new file and get this error:

  ✖  1:1  This module could be parsed as a valid script.  import/unambiguous

* For a module, add an `import` or `export` declaration to make the
  file unambiguously a module (the empty `export {};` declaration
  suffices).
* For a script, add the file to the xo overrides section of
  package.json that marks it "sourceType": "script", and add a 'use
  strict' declaration.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

app/renderer/js/utils/system-util.ts

@@ -1,4 +1,3 @@
-'use strict';
 import { remote } from 'electron';
 
 import os from 'os';