Replace lodash 'isPlainObject' usage with is-plain-object library

[jayaddison]

Q	A
Patch: Bug Fix?	No
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	No
Any Dependency Changes?	Yes
License	MIT

As per feedback in #11789 (comment), this is another instance of a lodash function that can be replaced by a standalone library.

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[jayaddison]

The two implementations seem to differ when determining whether an object created by using Object.create(null) is a plain object:

> isplainobject_standalone = require('is-plain-object');
[Function: isPlainObject]
> isplainobject_lodash = require('lodash/isPlainObject');
[Function: isPlainObject]
> var x = Object.create(null);
undefined
> isplainobject_standalone(x);
false
> isplainobject_lodash(x);
true

[TrySound]

Fixed this. Try v4
https://github.com/jonschlinkert/is-plain-object/releases/tag/v4.0.0

[jayaddison]

(switching to draft because I'm not sure what factors should determine the approach between this and #11859)

[kaicataldo]

Any reason we can't just use lodash.isplainobject instead? I don't think we need to get rid of lodash entirely, but it would be great if we could just install the utils we're using individually.

[TrySound]

lodash.* packages are deprecated and will not be supported in lodash v5.

[kaicataldo]

Ah, okay, that's a very good reason 👍

[jayaddison]

Closing given a marginal preference for #11859 instead.