T15774: Celerity: javascript minification issue with template literals (backticked strings)

Test Plan:
This archive contains a small demo: {F2142567}
When installed, an application named D25571 appears under "More Applications".
When you start it, you will see 2 messageboxes: one should be a single lined text, the other a multilined text.

Steps:
1) unpack archive in some directory
2) add ext-D25571/src/ path to load-libraries in local.json
3) bin/arc liberate
4) bin/cache purge --all
5) bin/celerity map
6) restart httpd
7) start D25571 application from More Applications

Also:

arc unit ./src/applications/celerity/__tests__/CelerityResourceTransformerTestCase.php

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: avivey, aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15774

Differential Revision: https://we.phorge.it/D25571

externals/JsShrink/jsShrink.php

···
 function jsShrink($input) {
 	return preg_replace_callback('(
 		(?:
-			(^|[-+\([{}=,:;!%^&*|?~]|/(?![/*])|return|throw) # context before regexp
+			(`(?:\\\\.|[^`\\\\])*`) # template literal
+			|(^|[-+\([{}=,:;!%^&*|?~]|/(?![/*])|return|throw) # context before regexp
 			(?:\s|//[^\n]*+\n|/\*(?:[^*]|\*(?!/))*+\*/)* # optional space
 			(/(?![/*])(?:
 				\\\\[^\n]
···
 
 function jsShrinkCallback($match) {
 	static $last = '';
-	$match += array_fill(1, 5, null); // avoid E_NOTICE
-	list(, $context, $regexp, $result, $word, $operator) = $match;
-	if ($word != '') {
+	$match += array_fill(1, 7, null); // avoid E_NOTICE
+	list(, $template, $context, $regexp, $result, $word, $operator) = $match;
+	if ($template) {
+		$result = $template;
+	} elseif ($word != '') {
 		$result = ($last == 'word' ? "\n" : ($last == 'return' ? " " : "")) . $result;
 		$last = ($word == 'return' || $word == 'throw' || $word == 'break' || $word == 'async' ? 'return' : 'word');
 	} elseif ($operator) {