Biomes can be inlined in biome sources and flat world presets

In biome sources and flat world presets, an inlined biome can be defined in place of a biome id reference. This doesn’t work as the game will crash if attempting to join a dimension with an inlined biome, and I imagine this being validated at all is the bug. This bug has existed since at least 1.16.2.

Steps to Reproduce

1. Install the attached datapack. It adds an apollo:the_sift dimension, with the biome source being fixed with an inline biome input.

2. Once in the world, run /execute in apollo:the_sift run tp @s ~ ~ ~

3. Crash.

Expected Behavior

The datapack would fail to validate if the biome was attempted to be inlined.

Actual Behavior

The datapack accepts the inlined biome and creates the dimension, even though if will crash when trying to enter it.

The Fix

Biome#CODEC needs to set the allowInline boolean to false.