breaking: remove buttonProps from experimental remote form functions

[dummdidumm]

With type-safe forms via myForm.fields the myForm.buttonProps API is an outlier. It allows you to apply a completely different form action onto an existing form. That means you lose all type safety you've gained. We therefore remove it in favor of as('submit', 'value') - instead of buttonProps you declare a new field on your schema on which you can decide which logic to execute:

// +page.svelte
<script>
-  import { login, register } from './login.remote';
+  import { login} from './login.remote';
</script>

<form {...login}>
  <!-- ... -->
  <button>login</button>
-  <button {...register.buttonProps}>register</button>
+  <button {...login.fields.action.as('submit', 'register')}>register</button>
</form>

// login.remote.ts
import { form } from '$app/server';

export const login = form(
-  v.object({ username: v.string(), _password: v.string() })
+  v.object({ username: v.string(), _password: v.string(), action: v.optional(v.picklist(['login', 'register'])) })
  (payload) => {
+    if (payload.action === 'register') {
+      // register...
+    } else {
      // login...
+    }
  }
);

-export const register = form(
-  v.object({ username: v.string(), _password: v.string() })
-  (payload) => {
-    // register...
-  }
-);

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