Manifest file with server cores as keys

[Arcadi4]

Currently, we adopt a []core + []package model. A simplified outline of the design:

cores:
  - minecraft@x.x.x
  - fabric@x.x.x
  - ...
packages:
  - carpet@x.x.x
  - servux@x.x.x
  - ...

This is an intuitive but sub-optimal design:

• It does not build a stable connection between core (environments) and their hosted packages.
• It's very difficult to make server cores' attributes heterogeneous.

Here is a proposed design we believe to be better:

global-option-1: ...
global-option-2: ...
global-option-3: ...

fabric:
  version: x.x.x
  packages:
    - fabric-api@x.x.x
    - carpet@x.x.x

mcdr:
  version: x.x.x
  packages:
    - mirrormcsmcdr@x.x.x
    - prime-backup@x.x.x

See also: #97