[hub] Export HUB_URL constant for consumers (#1489)

This PR exports the `HUB_URL` constant from the hub package so that
consumers can directly access it in their applications.

## Changes

- Added `export { HUB_URL } from "./consts";` to
`packages/hub/src/index.ts`
- Added a "Constants" section to the README.md explaining the exported
HUB_URL constant and how to use it

## Usage Example

```ts
import { HUB_URL } from "@huggingface/hub";

console.log(HUB_URL); // https://huggingface.co

// Can be useful when constructing URLs or checking against the default URL
const customHubUrl = process.env.CUSTOM_HUB_URL || HUB_URL;
```

This is useful for applications that need to reference the default hub
URL or construct hub-related URLs in their code, without having to
define the constant themselves.

Fixes #1336.

> [!WARNING]
>
> <details>
> <summary>Firewall rules blocked me from connecting to one or more
addresses</summary>
>
> #### I tried to connect to the following addresses, but was blocked by
firewall rules:
>
> - `hg.mozilla.org`
>   - Triggering command: `node ./dist/install.js ` (dns block)
> - `hub-ci.huggingface.co`
> - Triggering command: `node
/home/REDACTED/work/huggingface.js/huggingface.js/node_modules/.bin/../vitest/vitest.mjs
run ` (dns block)
> - `huggingface.co`
> - Triggering command: `node
/home/REDACTED/work/huggingface.js/huggingface.js/node_modules/.bin/../vitest/vitest.mjs
run ` (dns block)
>
> If you need me to access, download, or install something from one of
these locations, you can either:
>
> - Configure [Actions setup
steps](https://gh.io/copilot/actions-setup-steps) to set up my
environment, which run before the firewall is enabled
> - Add the appropriate URLs or hosts to my [firewall allow
list](https://gh.io/copilot/firewall-config)
>
> </details>


---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
tips](https://gh.io/copilot-coding-agent-tips) in the docs.

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: julien-c <[email protected]>

Author: Copilot

packages/hub/src/index.ts

@@ -18,6 +18,7 @@ export type {
 	SpaceStage,
 } from "./types/public";
 export { HubApiError, InvalidApiResponseFormatError } from "./error";
+export { HUB_URL } from "./consts";
 /**
  * Only exported for E2Es convenience
  */