New entity: info_particle_system_coordinate, plus infinite cooldown functionality for prop_interactable

[arbabf]

info_particle_system_coordinate is a new entity devised by the Raising The Bar: Redux team, and implemented by Iridium77 and me. The intended use-case for this is the ability to create particle systems with multiple control points, without having to specify entities - instead, using Vectors, thus keeping the entity count low for larger maps. Compared to placing info_targets for each control point, this reduces total entity count taken up by a regular info_particle_system by up to 64-fold.

Additionally, because of a lack of one in prop_interactable, there is now an infinite cooldown functionality. Mappers can set their cooldowns to -1 to never allow the prop to be used again if pushed while unlocked. If pushed while locked, however, a default cooldown of 1 second applies.

The relevant FGD entries are attached.

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Does this PR close any issues?

• None.

PR Checklist

• My PR follows all guidelines in the CONTRIBUTING.md file
• My PR targets a develop branch OR targets another branch with a specific goal in mind

fgd.txt

[Blixibon]

The spacing changes make this really hard to read, but this otherwise looks good to me