Add more customization options to the language switcher plugin

[amirhhashemi]

This PR adds several new customization options to the language switcher plugin:

1. Users can now customize the label of the toggle button.
2. Users can style the switcher through Expressive Code's style settings.
3. The new conversions option provides greater control over the plugin's default behavior. Most notably, it allows users to add support for custom languages (like Svelte, Vue, etc.) if they wish.

It also includes a few improvements and fixes:

1. Styles are now conditionally loaded only when the plugin is enabled. Previously, they were always included as part of the global index.css file.
2. The plugin is now exported through the solidbase/plugins path. This change is important because it gives users access to helper functions used within the plugin, useful when customizing conversions.

These changes should be backward compatible.

I'll be honest, getting this all to work required a few less-than-ideal compromises, and I'm not entirely happy with the code quality. The implementation ended up being a bit more complex than I expected. To be frank, these features don't add much value for the Solid docs project, so I completely understand if you decide not to merge this PR (or only parts of it).

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