feat: Implement a default main generator (API first)

[sthzg]

The main generator should take a config file and run all generations defined in it. After the API implementation it should be made available as the root jsugen CLI command (i.e. executed when no subcommand is given).

Motivation:

a) Clients should be able to express their required generators in a config file format
b) Running multiple generators should not require any client-side wrapper scripts
c) Users should be able to maintain their own generators rather than relying on those implemented in the package
d) Generation should not require installing the package and its dependencies to the local dependency tree (hint: convenience with npx)

Early config format draft:

definitions:
  - # input format
    source: 'json-schema' 
    # source files as string or array of strings in glob format
    files: '**/*.schema.json' 
    # array of generator plugins to run (dynamically resolved to allow user plugins)
    generators: 
      - enums
      - selector-functions
      - member-names
      - lodash-object-paths
    # target directory to store generated files
    output: 
      baseDirectory: './'

Working branch:
https://github.com/sthzg/jsugen/tree/feature/59

[sthzg]

Closed with #62