Add a base class to `external` for "Multi-File Objects"

[snake-biscuits]

Plan

• come up with a name
• create base class
  • initialisers
    • .from_archive
    • .from_file
  • methods
    • .mount_file
    • .unmount_file
  • everything else I forgot to list here

Initial Idea

Comment in #209:

for importing, all SceneDescriptions will need to be multi file objects
basically every format will have attached files:

• .mtl
• .usd[acz]
• .bin
• textures

we already have a system for mounting and unmounting files for:

• BspClasses
• ExternalLumpManager
• ArchiveClasses
• DiscClasses

might as well move that into a base class to reduce duplicate code.

[snake-biscuits]

external.MultiFileObject is overly verbose
external.MFO is unclear

neither name properly conveys use
.from_file() should target a core file
this class simply allows for attaching other relevant files

external.MainFile?
external.ParentFile?

external.CoreFile.

[snake-biscuits]

though now the module name seems off
external.File made sense
but now it's all about file operations
clustering files under a primary file which references others

maybe move to:
bsp_tool.file.CoreFile
bsp_tool.file.LinkedFile