Preserve type in first for OneTo (#56263)

With this PR,
```julia
julia> first(Base.OneTo(10), 4)
Base.OneTo(4)
```
Previously, this would have used indexing to return a `UnitRange`. This
is probably the only way to slice a `Base.OneTo` and obtain a
`Base.OneTo` back.

Author: jishnub

base/range.jl

@@ -850,6 +850,11 @@ first(r::OneTo{T}) where {T} = oneunit(T)
 first(r::StepRangeLen) = unsafe_getindex(r, 1)
 first(r::LinRange) = r.start
 
+function first(r::OneTo, n::Integer)
+    n < 0 && throw(ArgumentError("Number of elements must be non-negative"))
+    OneTo(oftype(r.stop, min(r.stop, n)))
+end
+
 last(r::OrdinalRange{T}) where {T} = convert(T, r.stop) # via steprange_last
 last(r::StepRangeLen) = unsafe_getindex(r, length(r))
 last(r::LinRange) = r.stop

test/ranges.jl

@@ -1539,6 +1539,9 @@ end
         @test size(r) == (3,)
         @test step(r) == 1
         @test first(r) == 1
+        @test first(r,2) === Base.OneTo(2)
+        @test first(r,20) === r
+        @test_throws ArgumentError first(r,-20)
         @test last(r) == 3
         @test minimum(r) == 1
         @test maximum(r) == 3
@@ -1570,6 +1573,9 @@ end
         @test findall(in(2:(length(r) - 1)), r) === 2:(length(r) - 1)
         @test findall(in(r), 2:(length(r) - 1)) === 1:(length(r) - 2)
     end
+    let r = Base.OneTo(Int8(4))
+        @test first(r,4) === r
+    end
     @test convert(Base.OneTo, 1:2) === Base.OneTo{Int}(2)
     @test_throws ArgumentError("first element must be 1, got 2") convert(Base.OneTo, 2:3)
     @test_throws ArgumentError("step must be 1, got 2") convert(Base.OneTo, 1:2:5)