EntitySpawnsCheatSheetFabric.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;

import com.google.common.collect.ImmutableMap;

import net.minecraft.entity.EntityType;
import net.minecraft.entity.SpawnGroup;
import net.minecraft.util.registry.BuiltinRegistries;
import net.minecraft.world.biome.Biome;
import net.minecraft.world.biome.SpawnSettings;

public class EntitySpawnsCheatSheetFabric {
    
    //Iterate through all registered biomes in the WorldGenRegistries.
    public static void addSpawnEntries() {
        for (Biome biome : BuiltinRegistries.BIOME) {
            if (biome.getCategory() != Biome.Category.NETHER && biome.getCategory() != Biome.Category.THEEND && biome.getCategory() != Biome.Category.NONE) {
                addMobSpawnToBiome(biome, SpawnGroup.MONSTER, new SpawnSettings.SpawnEntry(EntityType.ENDERMAN, 100, 100, 100));
            }
        }
    }

    //Add the normal info along with a list of "Spawners"
    public static void addMobSpawnToBiome(Biome biome, SpawnGroup classification, SpawnSettings.SpawnEntry... spawners) {
        convertImmutableSpawners(biome);
        //Copy the list of spawners that already exist for the given biome.
        List<SpawnSettings.SpawnEntry> spawnersList = new ArrayList<>(biome.getSpawnSettings().spawners.get(classification));
        //Add all the spawners within our list.
        spawnersList.addAll(Arrays.asList(spawners));
        //Overwrite the list for the given classification with the old list and our new entries.
        biome.getSpawnSettings().spawners.put(classification, spawnersList);
    }

    //Convert the immutable map to a mutable HashMap in order for us to change the data stored in these maps
    private static void convertImmutableSpawners(Biome biome) {
        if (biome.getSpawnSettings().spawners instanceof ImmutableMap) {
            biome.getSpawnSettings().spawners = new HashMap<>(biome.getSpawnSettings().spawners);
        }
    }
}

Here are the Access Wideners you need for this

accessible    field    net/minecraft/world/biome/SpawnSettings    spawners    Ljava/util/Map;

mutable    field    net/minecraft/world/biome/SpawnSettings    spawners    Ljava/util/Map;

accessible    method    net/minecraft/entity/SpawnRestriction    register    (Lnet/minecraft/entity/EntityType;Lnet/minecraft/entity/SpawnRestriction$Location;Lnet/minecraft/world/Heightmap$Type;Lnet/minecraft/entity/SpawnRestriction$SpawnPredicate;)V