[gtksourceview] test: add JS BigInt example

[Christian Hergert <chergert src gnome org>]

commit 502ee735269991b732d7302e958a24949f6ce1fb
Author: Nuno Martins <nunocastromartins protonmail com>
Date:   Tue Sep 24 09:15:00 2019 +0100

    test: add JS BigInt example

 tests/syntax-highlighting/file.js | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/tests/syntax-highlighting/file.js b/tests/syntax-highlighting/file.js
index 7897808a..85ba01ea 100644
--- a/tests/syntax-highlighting/file.js
+++ b/tests/syntax-highlighting/file.js
@@ -33,6 +33,12 @@ let binary2 = 0B00001111;
 let octal1 = 0o0123;
 let octal2 = 0O4567;
 
+// Big integers:
+let bigInt = 543789527895762347856234897532n;
+let bigIntZero = 0n;
+let bigIntConst = BigInt('1234513123123');
+312312n+5234523n-12n*3123n/4n%3231n**-123123n;
+
 // Template strings
 // ----------------
 // Template strings are delimited by back-ticks (grave accent) and