Enable Access to Current URL in XMDL Language

Introduce a `page.url` property within the XMDL language to allow scripts to retrieve the current document's URL. This feature will facilitate dynamic content generation based on the page's URL, enhancing flexibility and user experience.

**Proposed Implementation:**
- Implement a `page.url` property accessible within XMDL scripts to obtain the current URL.

**Use Case Example:**
```xmdl
@lua{>
local currentUrl = page.url
-- Additional Lua code utilizing currentUrl
<}
```

**Benefits:**
- Enables context-aware content rendering based on the current URL.
- Facilitates the creation of dynamic and interactive web pages.

**Additional Notes:**
This feature is essential for developers aiming to create responsive and dynamic content within the XMDL framework. Implementing access to the current URL will significantly enhance the language's capability to handle context-specific scenarios.