Preserve newlines on copy + paste

[JonShort]

Adresses #78

Currently when copy + pasting, empty lines are not preserved in the output, which could be unexpected for the user.

Changes

Adding a newline \n as the content for empty rows seems to help preserve newlines when copy + pasting; without adding any additional characters (e.g. whitespace) or changing the visuals.

I've tested this on macOS & windows, but haven't been able to test on a linux device

Tests

Local:

[x] macOS
[x] Windows
[ ] Unix

Unit tests:

• Updated to expect \n as content for empty lines

Snapshots:

• Updated as empty line spans are no-longer self-closing

Examples

I shimmed this functionality into the codesandbox example, so it can be tested:
https://codesandbox.io/s/prism-react-renderer-example-r8dl3

note - the line numbers example seems to paste with extra newlines even in the original example, so I don't think there's a regression there

Pasted from examples:

Current

(function someDemo() {
  var test = "Hello World!";
  console.log(test);
})();
return () => <App />;

Updated

(function someDemo() {
  var test = "Hello World!";
  console.log(test);
})();

return () => <App />;