Add ExplosiveMinecart ignite() and explode() overloads and add getIgniter() (#14097)

[WouterGritter]

Adds:

• ExplosiveMinecart#ignite(int fuseTime)
• ExplosiveMinecart#ignite(Entity igniter)
• ExplosiveMinecart#ignite(Entity igniter, int fuseTime)
• ExplosiveMinecart#explode(Entity entity)
• ExplosiveMinecart#explode(Entity entity, double power)
• ExplosiveMinecart#getIgniter()

ignite(int fuseTime) was oddly missing - an earlier Paper patch added support for this through NMS but it wasn't exposed through the API before. ExplosiveMinecart#setFuseTicks exists "If the fuse ticks are set to a non-zero value, this will ignite the explosive." (from its javadocs), but this does not play the TNT_PRIMED sound and also doesn't fire EntityIgniteEvent. Should we deprecate this method in this PR?

This PR closes #14097 (Add .getIgniter() to ExplosiveMinecart).

At first it seemed better to expose MinecartTNT's DamageSource ignitionSource, but this is always an explosion damage source, optionally with the igniter entity as its cause, that the minecart will use once exploding.

MinecartTNT#primeFuse is a bit odd as it consumes a DamageSource source, which is the source that did damage to the minecart that caused it to prime, though the type is discarded. Right now I've got this wrapped in another explosion source, which gets stripped down to its causing entity, then re-wrapped in a new explosion source (by vanilla code).

[Doc94]

Maybe i mess read something

[Doc94]

Use JSpecify (and include the NullMarked)

[WouterGritter]

@NullMarked is only used in the package-info.java's, and CommandMinecart also exists in this package with @NotNull / @Nullable annotations (jetbrains). Should I add @NullMarked to the package-info in this PR, and update CommandMinecart also? Seems a bit out of scope, but otherwise a @NullMarked in package-info would be misleading

[Doc94]

The idea is avoid add the Jetbrains nullable annotation and move if is possible and not require big reworks (that is for another instance), you can by the moment just add to the Interface and later move to package when need.

[WouterGritter]

Will do this for now, but this will be the first and only interface (or class) that is annotated by @NullMarked, the rest of the codebase only uses this in package-info.java

[WouterGritter]

Force-pushed the requested changes (MinecartTNT now has no changes, apart from the AT), including a second commit that switches the org.bukkit.entity.minecart over to @NullMarked (only a minor change in CommandMinecart, out-of-scope-ish, let me know if this should be reversed)

[Lulu13022002]

This doesn't target the right branch should be on main (then depending when it's merged it can be moved to ver/26.2)

[WouterGritter]

Rebased to main