feat: add functionality to serve offline fallback map

[achou11]

Towards #439

Stacked on #435

Implements a fastify plugin for serving a fallback map in the case of not having an available local "default" static map or being offline. The plugin accepts required options for specifying where the style definition lives and the directory containing the geojson sources (this provides flexibility for consumers that may not necessarily be using something like mapeo-offline-map, or happen to change the filesystem structure).

TODO:

• add documentation about the expected structure of the fallback map style definition and source files

[achou11]

added as a dev dependency but if we want to be more strict about using this specifically, can change to direct or peer dep.

wasn't too keen on the offline map being bundled as of the npm package so i figured it would make sense to let the consumer decide to add the map as a dependency in its own project

[EvanHahn]

LGTM other than my annoying nitpicks.