Add package-lock.json

In order to have 100% reproducible builds, Electron needs
package-lock.json. This is necessary because some dependencies affect
the build output (browserify for example, is used to generate the common
js environment for sandboxed renderers).

.gitignore

@@ -44,7 +44,6 @@
 /vendor/LICENSES.chromium.html
 node_modules/
 SHASUMS256.txt
-**/package-lock.json
 **/yarn.lock
 compile_commands.json
 .envrc