gen-dev: impl Num.neg for Dec,F32,F64

Dec negation was implemented across gen-dev, gen-llvm, gen-wasm as a call
to the compiled zig function `bitcode::DEC_NEGATE`.

f32 and f64 negation were implemented already for gen-llvm, gen-wasm.

for gen-dev x86_64, float negation is implemented by flipping the sign
bit, which means `xorps` for f32, and `xorpd` for f64

for gen-dev aarch64, there is conveniently a `fneg` instruction

crates/compiler/gen_wasm/src/low_level.rs

@@ -1624,6 +1624,7 @@ impl<'a> LowLevelCall<'a> {
                     }
                     F32 => backend.code_builder.f32_neg(),
                     F64 => backend.code_builder.f64_neg(),
+                    Decimal => self.load_args_and_call_zig(backend, bitcode::DEC_NEGATE),
                     _ => todo!("{:?} for {:?}", self.lowlevel, self.ret_layout),
                 }
             }