Preserves extra fields on re-init

[nickesc]

Fixes a bug that could cause fields to be lost when running init in a directory that already has a modpack.json file. If there are fields that aren't set during the init flow, it will discard them, including the scripts field.

With this hotfix:

• Merges existing modpack info with prompted answers so any extra keys present in the original file are preserved instead of being dropped
• Prevents accidental destruction of scripts, other custom fields

[nickesc]

looks good. bump version, generate docs and publish