[slint-lsp] add an action to open the documentation of the "selected" token

[abique]

Hi,

I'd love to be able to read the documentation of a Slint element, type, ... by using a VSCode / RustRover action.

Cheers, Alex

[ogoffart]

Thanks for filling the issue.

This kind of related to https://github.com/slint-ui/slint/issues/104

We should put the documentation in the .slint files so then the LSP could know the documentation. (For our builtin widget it's extra tricky because they are duplicated for each style. But we could just decide to document the fluent style and use that. Or use interface https://github.com/slint-ui/slint/issues/1870 )

[abique]

Opening the webpage would be enough for now.

[abique]

Sorry for #5083 I forgot about this issue!