Do not auto merge package-lock.json, but treat as text file

If package-lock is specified to be a binary file, it becomes impossible
to diff any changes. Since we only want to prevent git from merging
changes in the lockfile, we can be more specific and tell it that binary
handling is only required as merge strategy.

[skip ci]

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

.gitattributes

@@ -1,2 +1,2 @@
 /dist/* binary
-/package-lock.json binary
+/package-lock.json merge=binary