Implement "DropStrider" input on dropships

[Bronzehawk75]

Dropships and striders have inputs and animations for being dropped directly into combat, but the feature was never finished. This PR implements that functionality.

This reuses a lot of logic from Combine soldier drop-offs, so most of the basic setup for mappers is the same (an info_target landing point, OnFinishedDropoff after it's done, etc.)

A few notes:

• Dropships can now specify a template NPC for the strider it's carrying with the StriderTemplate field.
• If a strider is killed while being carried by a dropship, the dropship now fires the OnCrateShotDownBeforeDropoff output.
• The strider doesn't make any sounds or effects when it hits the ground. I've never done any modeling, but I'm pretty sure the correct way to do this would be to recompile the strider model with a new animation event on frame 100 of the deploy animation.

Test map: dropship_strider_test.zip

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Does this PR close any issues?

• No

PR Checklist

• My PR follows all guidelines in the CONTRIBUTING.md file
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