Is your feature request related to a problem? Please describe.

Currently we can access a single selected commit via {{.SelectedCommit.Sha}}.
However using the v key it's also possible to select multiple commits.
I.e. this allows to create a pull request from a range of commits or doing other types of automation that require access to multiple commits.

Describe the solution you'd like

A potential API could look like thi
s:
{{.SelectedCommits.StartSha}} {{.SelectedCommits.EndSha}}

Describe alternatives you've considered

Can't think of a different way

From my understanding this needs to extend

https://github.com/jesseduffield/lazygit/blob/master/pkg/gui/services/custom_commands/session_state_loader.go#L203

The only thing that is unclear to me is what should happen if the user has not selected anything?