Add benchmark/Project.toml so the benchmark suite resolves StableRNGs

The `Benchmarks` CI job errored with `ArgumentError: Package StableRNGs not
found in current path`. AirspeedVelocity builds a temporary environment for
the benchmarked revision and, when a `benchmark/Project.toml` is present in
that revision's checkout, copies it in as the environment (it auto-adds
BenchmarkTools/JSON3 and the package under test, but nothing else). There was
no such file, so `using StableRNGs` in `benchmark/benchmarks.jl` could not
resolve.

Add `benchmark/Project.toml` declaring StableRNGs. Verified by replicating
AirspeedVelocity's env construction (copy the file, develop the package, add
BenchmarkTools/JSON3): the unmodified suite loads, builds all groups, and runs
a benchmarkable.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FvBXVKGujjeaCB3iLwsDeG

Author: ChrisRackauckas

benchmark/Project.toml

@@ -0,0 +1,5 @@
+[deps]
+StableRNGs = "860ef19b-820b-49d6-a774-d7a799459cd3"
+
+[compat]
+StableRNGs = "1"