Refactor how wasm features are spelled/enabled (#9168)

Use the new `WasmFeatures::wasm2()` constructor to avoid having to
indefinitely list old proposals where this can get updated as newer
versions of the specification are released that Wasmtime fully
implements. Additionally slightly refactor how conditional enabling of
features work and instead use the `set` function to guarantee setting
the requisite feature to the desired value.

crates/wasmtime/src/config.rs

@@ -1785,32 +1785,26 @@ impl Config {
     /// `wasm_*` methods on `Config` are applied. Everything is then validated
     /// later in `Config::validate`.
     fn features(&self) -> WasmFeatures {
-        let mut features = WasmFeatures::empty();
+        // Wasmtime by default supports all of the wasm 2.0 version of the
+        // specification.
+        let mut features = WasmFeatures::wasm2();
 
         // On-by-default features that wasmtime has. Note that these are all
         // subject to the criteria at
         // https://docs.wasmtime.dev/contributing-implementing-wasm-proposals.html
-        features |= WasmFeatures::FLOATS;
-        features |= WasmFeatures::MULTI_VALUE;
-        features |= WasmFeatures::BULK_MEMORY;
-        features |= WasmFeatures::SIGN_EXTENSION;
-        features |= WasmFeatures::MUTABLE_GLOBAL;
-        features |= WasmFeatures::SATURATING_FLOAT_TO_INT;
         features |= WasmFeatures::MULTI_MEMORY;
-        features |= WasmFeatures::SIMD;
         features |= WasmFeatures::RELAXED_SIMD;
         features |= WasmFeatures::TAIL_CALL;
         features |= WasmFeatures::EXTENDED_CONST;
-        features |= WasmFeatures::REFERENCE_TYPES;
-        if cfg!(feature = "gc") {
-            features |= WasmFeatures::GC_TYPES;
-        }
-        if cfg!(feature = "threads") {
-            features |= WasmFeatures::THREADS;
-        }
-        if cfg!(feature = "component-model") {
-            features |= WasmFeatures::COMPONENT_MODEL;
-        }
+
+        // Set some features to their conditionally-enabled defaults depending
+        // on crate compile-time features.
+        features.set(WasmFeatures::GC_TYPES, cfg!(feature = "gc"));
+        features.set(WasmFeatures::THREADS, cfg!(feature = "threads"));
+        features.set(
+            WasmFeatures::COMPONENT_MODEL,
+            cfg!(feature = "component-model"),
+        );
 
         // From the default set of proposals remove any that the current
         // compiler backend may panic on if the module contains them.