fix: add missing wasm_toolchain bzl_library target

Add the missing bzl_library target for wasm_toolchain that is referenced
by the wasm/extensions.bzl dependency.

The extensions.bzl file was trying to depend on //toolchains:wasm_toolchain
but this target was not declared in the BUILD.bazel file. This commit
adds the missing bzl_library declaration to resolve the build error.

Changes:
- Add bzl_library target for wasm_toolchain.bzl in toolchains/BUILD.bazel
- Import @bazel_skylib//:bzl_library.bzl load statement
- Set proper visibility for the bzl_library target

Author: Rules WASM Component

toolchains/BUILD.bazel

@@ -1,9 +1,18 @@
 """Toolchain type definitions"""
 
+load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
+
 package(default_visibility = ["//visibility:public"])
 
 # Toolchain type for WebAssembly tools
 toolchain_type(
     name = "wasm_tools_toolchain_type",
     visibility = ["//visibility:public"],
+)
+
+# Bzl library for toolchain implementation
+bzl_library(
+    name = "wasm_toolchain",
+    srcs = ["wasm_toolchain.bzl"],
+    visibility = ["//visibility:public"],
 )