Allow editing multiline prompts with external text editor (#429)

- Adds a new `Preferences` struct to the config, initially only containing `Editor: bool (default false)`.
  This struct will be serialized to configs once there is a first tea induced change to the config (eg `tea login default <name>` or `tea login add`).
- Use external editor for all multiline prompts if preferred.

We already had a function for starting a texteditor for diff reviews; it does not really make sense to replace it with `survey.Editor`, as there is a big interface mismatch: survey expects strings as inputs, while our diff functions operate on files,

fixes #424

Co-authored-by: Norwin <git@nroo.de>
Reviewed-on: https://gitea.com/gitea/tea/pulls/429
Reviewed-by: Andrew Thornton <art27@cantab.net>
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Norwin <noerw@noreply.gitea.io>
Co-committed-by: Norwin <noerw@noreply.gitea.io>

modules/interact/comments.go

@@ -11,6 +11,7 @@ import (
 	"code.gitea.io/sdk/gitea"
 	"code.gitea.io/tea/modules/context"
 	"code.gitea.io/tea/modules/print"
+
 	"github.com/AlecAivazis/survey/v2"
 	"golang.org/x/crypto/ssh/terminal"
 )