playerforms.xml and modded playerforms support

Transformations are a mechanic present in isaac since the flash era
giving modders the ability to add new playerforms and the tools to add playerform progress to existing items/items from other mods for cross compatibility are needed to make any vanilla-like transformation focused mod and help general content packs include their own seamlessly.

features requested:

xml:
playerforms.xml working in the content folder to add modded playerforms similiarly to how items.xml works
Assign the same "id" to a type="null" item in items.xml to have that playerform apply automatically the "null" item
collectibles and trinkets in items.xml having the same tag as a modded playerform grant progress towards the playerform when held
optional: ability to define playerform milestone to reach instead of being hardcoded to 3

Functions:
Isaac.GetPlayerFormIdByName(string PlayerFormName) returning the real Id of the PlayerForm
Isaac.GetConfigTagByName(string ItemConfigTag) returning the tag's bitflag
ItemConfigItem:AddTags(int Tags) to add transformation tags to items with lua, so mods can add their own tags to vanilla items/modded items from other mods

Start by comparing the requested playerforms.xml behavior with the existing items.xml model and review the listed Lua entry points: Isaac.GetPlayerFormIdByName, Isaac.GetConfigTagByName, and ItemConfigItem:AddTags. Done means modded playerforms, item and trinket progress tags, and the requested Lua APIs work together, including any configured milestone behavior.