threads: add feature flag for threads (#1932)

Signed-off-by: Anuraag Agrawal <anuraaga@gmail.com>

experimental/features.go

@@ -0,0 +1,17 @@
+package experimental
+
+import "github.com/tetratelabs/wazero/api"
+
+// CoreFeaturesThreads enables threads instructions ("threads").
+//
+// # Notes
+//
+//   - This is not yet implemented by default, so you will need to use
+//     wazero.NewRuntimeConfigInterpreter
+//   - The instruction list is too long to enumerate in godoc.
+//     See https://github.com/WebAssembly/threads/blob/main/proposals/threads/Overview.md
+//   - Atomic operations are guest-only until api.Memory or otherwise expose them to host functions.
+//   - On systems without mmap available, the memory will pre-allocate to the maximum size. Many
+//     binaries will use a theroetical maximum like 4GB, so if using such a binary on a system
+//     without mmap, consider editing the binary to reduce the max size setting of memory.
+const CoreFeaturesThreads = api.CoreFeatureSIMD << 1