Flowers: Make flower spread a public function to enable override

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Remove search for 'group:flora_block'
Enable flora spread on 'dirt with dry grass'
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Simplifies the function for most users and enables function override for mods, such as plantlife, which use the 'flora_block' group.

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Tested.
But discovered issues: dry grasses are flora but do not spread, i will change the code to allow spread on dirt with dry grass. This may allow green grass and flowers to spread onto savanna, and allow dry grasses to spread onto green grass, will test to see if this is a problem. Spread tends to self-limit it's distance so hopefully we can avoid non-native decorations spreading to far.

Or now grasses have groups we can use groups to control spread on certain surfaces.

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Updated. Dry grass can now spread in savanna.
Coded the simple way which allows some spread into neighbour biomes, possibly an interesting side effect.

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In testing this was the most cross-biome spread i observed, the first flower is just above the crosshair, this screenshot is when the spread had setled down and stabilised. Possibly a nice effect?
Here decorations were disabled, in actual mapgen the surrounding grasses would limit spread to less than this.
This commit does mean that flora can spread on non-native surfaces.

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I'll merge this on one approval due to it being semi-trivial, freeze being close and other devs being absent.

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ab7b7c7