Don't assume 1-based indexing.

[gafter]

From https://github.com/gafter/Rematch2.jl/pull/4#issuecomment-1523123644

When dealing with arrays, the generated code assumes one-based indexing which is fine for most cases, but to support packages like OffsetArrays, we should switch to firstindex(a) / lastindex(a) instead of 1 and length(a) etc.