After several requests for an API, the latest update added an API of sorts, for other mod makers to interface with this mod.

Enables/disables the mechanic where immortal hearts turn into flies for The Lost/Keeper. By default this is enabled.

Returns a table that contains the immortal heart data for all players. For more info on how this table is structured, see the comments in main.lua (line 90).

Spawns an immortal heart at the specified position and velocity.
If the arguments are nil, the player position and a 0 velocity are used.

Returns the pickup variant for immortal hearts. This variable is the result of the Immortal Hearts mod calling Isaac.GetEntityVariantByName("Immortal Heart").

Returns the sound ID for the immortal heart pickup sound. This variable is the result of the Immortal Hearts mod calling Isaac.GetSoundIdByName("ImmortalHeart_Pickup").

Returns the sound ID for the immortal heart refill sound. This variable is the result of the Immortal Hearts mod calling Isaac.GetSoundIdByName("ImmortalHeart_Refill").

Example usage:
This example code will spawn an immortal heart at player 0's position. ImmortalHeartsAPI is a global variable, and you should check if it exists before trying to use it.