fix(@turf/line-slice-along): interpolate altitude on sliced endpoints

[JSap0914]

Closes #3007

Bug

When PR #2852 updated destination() to pass through its origin's altitude, lineSliceAlong began emitting wrong elevations for the interpolated start/stop vertices it creates whenever a slice boundary falls in the middle of a segment.

The function calls destination(coords[i], overshot, bearing) — where coords[i] is the far end of the current segment — so the copied altitude was always that of the far endpoint, regardless of where the slice actually fell.

Reproduction (from issue #3007):

const line = turf.lineString([[0, 0, 0], [0, 0.0009, 100]]); // ~100 m, elev 0→100 m
const sliced = turf.lineSliceAlong(line, 0, 50, { units: 'meters' });
// sliced endpoint elevation: 100 ← wrong; expected ≈ 50

Fix

After computing the 2-D destination, calculate the fractional position of the interpolated vertex along its host segment and derive the altitude as a linear interpolation between the two segment endpoints:

fraction = 1 - |overshot| / segDist
altitude = from[2] + fraction * (to[2] - from[2])

If either endpoint has no altitude the altitude component is removed from the output, preserving the 2-D behaviour that existed before #2852.

Verification

Three new tests added to packages/turf-line-slice-along/test.ts:

Test	Before fix	After fix
Stop vertex altitude on midpoint slice (expected ≈50 m)	❌ 100	✅ 49.96
Start vertex altitude when slice starts mid-segment (expected ≈25 m)	❌ 100	✅ 24.98
2-D line produces 2-D interpolated vertex	✅ passes	✅ passes

All 21 pre-existing tests continue to pass unchanged.

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