pb: copy to the clipboard in the browser

Instead of erroring, the wasm `pb` branch now writes the inner command's data to the
host page's clipboard via navigator.clipboard (new hel_clipboard_write host hook).
Runs inside the Enter-keypress handler, so the clipboard user-gesture requirement is
met. Native pbcopy/xclip path unchanged.

helwasm/pkg/helwasm.js

@@ -142,6 +142,9 @@ function __wbg_get_imports() {
             const ret = arg0.getTimezoneOffset();
             return ret;
         },
+        __wbg_hel_clipboard_write_efa8177dd67c65ec: function(arg0, arg1) {
+            hel_clipboard_write(getStringFromWasm0(arg0, arg1));
+        },
         __wbg_hel_get_password_271a2beac04c29db: function(arg0, arg1, arg2) {
             const ret = hel_get_password(getStringFromWasm0(arg1, arg2));
             const ptr1 = passStringToWasm0(ret, wasm.__wbindgen_malloc, wasm.__wbindgen_realloc);