[FR] Add `Spring.SpawnWeaponExplosion`

### User story

I am a newbie modder who wants to spawn an explosion of an existing weapon, that's it.

### Deficiencies of existing interfaces

`Spring.SpawnExplosion` exists but it has a bad interface for this. It defaults to 0 aoe and 0 damage and 0 edgeeffectiveness and 0 literally everything. It doesn't require to pass a weaponDefID (so you can get weird ghost explosions with no FX), but even if you do pass one, it still doesn't affect the default damage/aoe/ee/etc so you need to replicate them manually. The flat (non-table) version also has the weaponDefID as far as the 16th parameter, being considered less important than such things as `gfxMod` and `impactOnly`. Using this function is tedious and error-prone.

`Spring.CreateProjectile` combined with either `ttl=0` or `Spring.DestroyProjectile` seems to spawn an explosion with the correct damage etc but actually does more than that; it triggers `gadget:ProjectileCreated/Destroyed` which may make assumptions and have side effects.

### Proposal

Add `Spring.SpawnWeaponExplosion(weaponDefID, x, y, z, rest of args)`. The `weaponDefID` is mandatory and affects the default damage/aoe/ee/etc. The rest of args are in a similar format as `SpawnExplosion` (ie. table or flat) but the flat version has them in a more sensible order (which is a vague term and may be discussed).

Start by comparing the existing Spring.SpawnExplosion and Spring.CreateProjectile interfaces, including their documented arguments and side effects. Define the new Spring.SpawnWeaponExplosion entry point around a mandatory weaponDefID and weapon-derived defaults; done means it supports table and flat forms without triggering projectile lifecycle callbacks. The flat argument order remains open for discussion.

Written by the indexing model from the issue text.