feat: add functionality to serve statically hosted maps

[achou11]

Closes #71

Ports over the "legacy" functionality of providing map style assets based on a static directory.

Notes:

• currently does NOT match against the /styles/:id/:z/:x/:y, which mapeo-server does. it's functionally similar to the /styles/:id/tiles/:tileId/:z/:x/:y, which this implementation does support. is it okay to assume that we control the tile urls that are referenced in the style.json files that we work with? the only reason i can imagine this additional route match exists is to account for potentially uncontrolled style.json files, but not entirely I'm missing something obvious 🤔

TODO:

• [x] ~tests~
• [ ] ~support for z/x/y tile directory? not sure if we actually supported this for legacy Mapeo maps so would be good to confirm before trying to get it to work.~

[achou11]

Closing this for now as we'll add this mapeo core next directly instead