feat(monorepo): split indexer into apps and packages

[tk-o]

Related to #52

Goals

• easy exporting for ponder schema to support the upcoming @ponder/client feature
  • ponder plugins to be split into packages
    • cannot be done as ponder app requires all handlers to be defined within ponder's rootDir
    • rootDir is a single location, hence, having plugins in separate directories won't work
  • ponder-shema package to host ponder.schema.ts
• ensnode-common package to host all primitives that can be used outside of ponder app context, for example, on the client side
• opt-in to using the Subgraph-compatible middleware
  • the Subgraph middleware will have its own package
• single directory apps to host all runnable modules
  • ensnode ponder app will be the first module here
  • ensnode app will import ponder schema & config from the ponder-bootstrap package
    • the ponder app expects these files to be loaded from a single rootDir — having ponder plugins in play, also place in the rootDir, prevents extracting ponder.config.ts & ponder.schema.ts into a separate package