Consolidate localisation config

[stephenwf]

At the moment we have the locales (dynamically imported) from the BaseExtension: https://github.com/UniversalViewer/universalviewer/blob/67ada49f6d866a51ab173d7166f6ddf04fffb471/src/content-handlers/iiif/extensions/uv-default-extension/Extension.ts

But then, some of the classes that extend from this base redefine the locales, without modification. This makes it difficult to add a new locale (e.g. #874)

Examples: https://github.com/UniversalViewer/universalviewer/blob/67ada49f6d866a51ab173d7166f6ddf04fffb471/src/content-handlers/iiif/extensions/uv-pdf-extension/Extension.ts#L34-L40 https://github.com/UniversalViewer/universalviewer/blob/67ada49f6d866a51ab173d7166f6ddf04fffb471/src/content-handlers/iiif/extensions/uv-ebook-extension/Extension.ts#L39-L45 https://github.com/UniversalViewer/universalviewer/blob/d3b9da59719806ffde86110a0d824ed4e55892e9/src/content-handlers/iiif/extensions/uv-av-extension/Extension.ts#L34-L40

A fix for now would be to remove these from the extensions that already extend from the base - unless there is a different base configuration.