Add "peek definition"

[clason]

I'd be nice to add a LspPeekDefinition (insert bike-shedding here, name taken from VS Code that offers this) that works like LspDefinition, but instead of jumping to the location in a new buffer, only shows the location in a preview window (like LspHover uses; bonus points for floating window when that is implemented).

[thomasfaingnaert]

@clason Does #412 fulfill your needs?

[clason]

Yes, that's exactly what I had in mind, thank you!