Convert admin components that will retain legacy support

[emteknetnz]

There are react class component(s) that needs to be converted to react functional component(s).

• LiteralField

Only one in this PR because it's a demonstration of the keeping legacy support for a subset of admin form field components where the old class components in retained separately. I want to ensure this way of retaining support is agreed upon before doing the rest.

Note there are more components than this to convert, limiting number converted at a time to make peer review more manageable.

Acceptance criteria

• Component(s) are converted to a functional component using hooks
• Jest test coverage is increased
• All existing functionality is preserved
• Legacy class components are retained (if applicable i.e. is admin form field component)

PRs

• ENH Convert LiteralField to functional component with legacy support #2118
• ENH Add LegacyLiteralField to externals.js webpack-config#90
• DOC Add changelog for legacy react components developer-docs#882

[GuySartorelli]

@emteknetnz Might be better to do one like TextField that we know gets extended

[emteknetnz]

I choose LiteralField simply because it's such a simple component and nothing in core extends it (unlike TextField), that it meant the real focus of this PR could be how to do the webpack-config, changelog, etc, and basically not worry about the functional component refactor

[GuySartorelli]

Okay. We'll see where we get with it. It may be difficult to judge it without actually seeing how it affects components that are already extending the existing component though.

[emteknetnz]

I've got test code in #2118 which I used to validate that it seems to work.