Exposing `modelPlaceholder.lua`

[SirLich]

Relevant mod: https://github.com/SirLich/sapiens-plus-plus

This file doesn't exactly have very much exposed, including some critical methods like addModel.

These are the things I think I need, but I may have missed some:

• addModel
• getModelIndexForStandardRemaps (?)
• Remaps such as longBranchRemaps

Actually I'm sure I missed more. It's hard for me to know what's needed for exposure without the chance to create something based on it.

[mjdave]

fixed b20