Upgrade to Bevy 0.18.

[andriyDev]

This migration isn't done since we still need to wait for our dependencies to upgrade, but at least bevy_landmass itself is ready.

Also the migration was basically nothing this time! Quite nice.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.3%. Comparing base (2b478d3) to head (172f76d).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/bevy_landmass/src/debug.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/bevy_landmass/src/agent.rs	81.6% <ø> (-0.3%)	⬇️
crates/landmass/src/avoidance.rs	97.6% <100.0%> (ø)
crates/landmass/src/lib.rs	94.5% <100.0%> (ø)
crates/landmass/src/nav_data.rs	97.5% <100.0%> (ø)
crates/bevy_landmass/src/debug.rs	26.5% <0.0%> (ø)

... and 5 files with indirect coverage changes

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