Add Modlist files

### Enhancement Type

Improve an existing feature

### Describe the enhancement

similar to DATAPACKS_FILE, a similar variable for auto downloaded mods may be nice.
DATAPACKS_FILE has the neat function that if changed, i dont have to compose up and down to apply, i can just restart the container. e.g. with MODRINTH_PROJECTS, i need to recompose the container / stack.

MODRINTH_PROJECTS_FILE would be nice, since its changeable like the datapack file

personal gripe: i use version control and have my modrinth stuff in a modrinth.env file. while fine since no secrets, including .env files in git always icks me a bit

Locate the existing DATAPACKS_FILE handling and the MODRINTH_PROJECTS configuration entry point, then compare how each is loaded on container startup. The work is done when a MODRINTH_PROJECTS_FILE option provides equivalent file-based configuration and changes take effect after restarting the container without recomposing it.

Written by the indexing model from the issue text.