Add varying monster spawn

[marmichalski]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Add ability to declare more than one type of monster that will be respawned after death:

	<spawn centerx="103" centery="120" centerz="7" radius="5">
		<monsters x="-1" y="1" z="7" spawntime="180">
			<monster name="Rabbit" chance="70" />
			<monster name="Skunk" chance="30" />
		</monsters>
	</spawn>

This can be of course mixed with single monster node:

	<spawn centerx="103" centery="120" centerz="7" radius="5">
		<monster name="Troll" x="4" y="-1" z="7" spawntime="60" />
		<monsters x="-1" y="1" z="7" spawntime="180">
			<monster name="Rabbit" chance="70" />
			<monster name="Skunk" chance="30" />
		</monsters>
	</spawn>

Total chance calculation could be done a bit better and less error prone, but as it's just an extra feature, I'll leave that to be improved by someone who likes math. 😎

Or chance could be mandatory, then the problem of auto-calculating what's left will be gone.

Issues addressed: #3600

If nobody is interested in adding it to RME, I can do that too.

[marmichalski]

This is a little bit of a lie, since the set is not empty, but contains non existent monster(s) only 🤔

[DSpeichert]

Similar: #3280

[MillhioreBT]

I guess RME should be updated to allow multiple monsters to be stacked in a single spawn point and give each one a chance.
otherwise who would want to edit the spawn.xml file by hand? XD

[ghost]

[ranisalt]

If nobody is interested in adding it to RME, I can do that too.

It would be interesting to check if their contributors agree on the format, though is good for us

[ghost]

@marmichalski I think you will be the one responsible for the RME part 😛