Fix hooks file to be TypeScript source

In order for other TypeScript code to consume snabbdom's TypeScript
correctly, we must "export" the hooks interfaces just like we export
other types.

tsconfig.json

@@ -27,7 +27,7 @@
         "src/modules/style.ts",
         "src/h.ts",
         "src/htmldomapi.ts",
-        "src/hooks.d.ts",
+        "src/hooks.ts",
         "src/is.ts",
         "src/snabbdom.bundle.ts",
         "src/snabbdom.ts",