NPM Package JSON Lint Config: Remove is-plain-obj dependency

[aduth]

Previously: #7556

This pull request seeks to use Lodash's isPlainObject instead of the standalone is-plain-obj package. This is intended to consolidate to reduce the overall number of packages we depend upon for the project.

Testing Instructions:

Verify unit tests pass:

npm run test-unit

[nerrad]

Was is-plain-obj initially used due to concern about the unnecessary size of the 'lodash' dependency?

[aduth]

@nerrad It was originally part of the WordPress/packages repository, which didn't ever have Lodash as a dependency. I might imagine it could have been an argument in favor against the size of pulling in Lodash. In the current situation, we already have it, and this should serve as a net decrease in dependency load (though, as it turns out by transitive dependencies, the is-plain-obj is still brought in).