Support for custom prompts for AI sidebar which work on Youtube

[Gerenuk]

It is already possible to set customs prompts for the AI sidebar with the setting `browser.ml.chat.prompts.*`. With this one can easily call the chosen AI provider with information from the current website. Currently, this is usually done with the text selection.

However, AIs like Gemini are really good a making summaries from a URL alone, since it connects to Youtube directly. This is particularly useful for Youtube, where you'd otherwise had to copy the whole video subtitles. In that case, it would be great to pass the URL in the custom prompt.

It is partly possible with the above configuration, however, the available `%url%` variable does not consider the parameters after `?` in the URL. Therefore, it does not work with Youtube.

It would be great if there was a variable (like `%fullurl%` or so) which stands for the whole URL including all parameters. Then one could easily make custom prompts which would also work well on Youtube. This is particularly useful for summarization of Youtube videos (with a custom focus prompt).

[Jon]

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