gitattributes: Mark yarn.lock as "binary", i.e. suppress diffs.

This makes many diffs that touch our JS dependencies in package.json
much, much more readable by suppressing multi-thousand-line changes.
With that kind of change routine, it's clear this isn't a file people
are closely reviewing the text diffs in anyway.

.gitattributes

@@ -10,3 +10,4 @@
 *.png binary
 *.otf binary
 *.tif binary
+yarn.lock binary