Navigation scripts are not well-encapsulated

**Describe the bug**

The JS scripts used by the navigation pattern do not work properly when there are multiple navigation components on the screen. This introduces issues for #5165 , where we would like the nav component to be fully interactive in combined form, just as it is in singular form.

**To Reproduce**

Steps to reproduce the behavior:

1. Add multiple navigation components to the same page. Import the same navigation scripts in each component separately. A good way to do this is to checkout the [combined navigation PR](https://github.com/canonical/vanilla-framework/pull/5165) and then revert the [commit that disabled its interactivity](https://github.com/canonical/vanilla-framework/pull/5165/commits/86a5aab39e073b6c14a85b8441f7ca51a9ad2ca4).
2. Observe console error:
```
Uncaught SyntaxError: redeclaration of const initNavigationSliding
```

**Expected behavior**

Multiple imports of the navigation scripts should not cause errors. The component's script should be encapsulated such that there can be multiple indepedent navigation components on the same DOM.

**Further context**
[MM thread](https://chat.canonical.com/canonical/pl/tg7ch6pe5pf7zp4pw6brtpwzmh)

Start with the navigation scripts used by the navigation pattern and reproduce the error by importing them separately in multiple components, as described in the issue and combined navigation PR #5165. Confirm that repeated imports no longer cause the redeclaration error and that multiple navigation components remain independently interactive.

Written by the indexing model from the issue text.