Add support for type checked term_at<T> and term_at<T>(i)

[deathbeam]

First method is simple loop over terms to find term for T. Its slower than using index directly, but sometimes type safety is more preferred and when the cost is only on initialization.

Second method is same as term_at(i) but with type checked assert. For middle ground to still keep the safety but also performance.

This is mostly to mirror the C# binding PR where this is slightly more useful but maybe cpp would benefit too:

BeanCheeseBurrito/Flecs.NET#52

Also disclaimer, I dont rly work with cpp often so I might've easily missed something, but I made best effort to write some tests and at least run them and see if they pass :d

Example use:

Current method

world.System<Position, Velocity>()
  .term_at(1)
  .singleton();

Type checked (throws assert if term_at(1) isnt Velocity type)

world.System<Position, Velocity>()
  .term_at<Velocity>(1)
  .singleton();

Type checked + resolved (throws assert if there is no Velocity component):

world.System<Position, Velocity>()
  .term_at<Velocity>()
  .singleton();

[copygirl]

Would it make sense to have the parameter-less term_at named differently?

[deathbeam]

Maybe just .term<>() and then also add .term(flecs::entity_t component) to match other apis?

[deathbeam]

Renamed term_at to term. Also updated the logic to account for pairs properly

[deathbeam]

Looks like I cant use term as it fails on build here so changing back to term_at I guess:

https://github.com/SanderMertens/flecs/actions/runs/11327040474/job/31497292704?pr=1397