rax: fix JSX RaxFragment usage/export

Typescript 5.7 will fail unless usage of RaxFragment is explicit, with
an import of it and a `/** @jsxFrag RaxFragment */` pragma at the top of
the file. This PR adds that.

RaxFragment also needs to have an exported value. I wasn't sure what to
use here so I copied preact, since preact is used in the Typescript
handbook as an example:

```ts
export const RaxFragment: FunctionComponent<{}>
```

Notably, though, the preact source has a comment indicating that
*they're* not sure of the correct type either.

Here is the Typescript 5.7 PR that made this error appear:

microsoft/TypeScript#59933

types/rax/index.d.ts

@@ -211,6 +211,7 @@ declare namespace Rax {
     interface RaxNodeArray extends Array<RaxNode> {}
     type RaxFragment = {} | RaxNodeArray;
     type RaxNode = RaxChild | RaxFragment | RaxPortal | boolean | null | undefined;
+    export const RaxFragment: FunctionComponent<{}>
 
     /**
      * ======================================================================

types/rax/test/hooks.tsx

@@ -1,10 +1,12 @@
+/** @jsxFrag RaxFragment */
 import {
     ComponentRef,
     createContext,
     createRef,
     forwardRef,
     MutableRefObject,
     RaxNode,
+    RaxFragment,
     Ref,
     RefObject,
     useCallback,