Make the "edit this setting" phantom key-bindable

[FichteFoll]

With ST4, we can implement our on_navigate callback as a command and thus also allow people to bind a key to this command. To make this work properly, we would need an optional argument for the setting name (that is passed in the phantom href) with the fallback picking the setting name of the current line.

If there is no setting on the current line, we could either choose the preceding or the following setting, depending on whether the caret is inside a comment currently.

This was raised on Discord.