Remove package-lock.json from .gitignore files

The package-lock.json file is intended to be checked in to source
control in order to make builds reproducible; without
a package-lock.json file `npm install` becomes non-deterministic.

See e.g.
https://docs.npmjs.com/cli/v8/configuring-npm/package-lock-json:

> package-lock.json is automatically generated for any operations where
> npm modifies either the node_modules tree, or package.json. It describes
> the exact tree that was generated, such that subsequent installs are
> able to generate identical trees, regardless of intermediate dependency
> updates.
>
> This file is intended to be committed into source repositories, and
> serves various purposes:

.gitignore

@@ -12,7 +12,6 @@
 .sbt
 .npm
 .node-gyp
-package-lock.json
 */experiments
 node_modules
 */notes

impls/.gitignore

@@ -11,7 +11,6 @@
 .sbt
 .npm
 .node-gyp
-package-lock.json
 .elm
 */experiments
 */node_modules