feat(pack): add support for sourcing manifest scripts

Problem: No way for plugins to define hooks that would be executed
  during plugin lifecycle. Like after install/update or before
  update/delete.

Solution: Automatically source scripts defined in plugin manifest file
  (if any) after triggering corresponding `PackChanged{,Pre}` events.
  During sourcing make some termporary adjustments:
  - Current directory is set to plugin's root to simplify execution of
    CLI calls like `make build`.
  - Plugin's path is ensured to be inside 'runtimepath' to allow using
    `require('plugin-module')` inside manifest scripts.

  The reason to execute after triggering event is so that there is a
  possibility for users to execute code both before the script (exactly
  on event) and after the script (more-or-less via `vim.schedule` called
  on the event).

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