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/milestone_create.go

@@ -33,7 +33,11 @@ func CreateMilestone(login *config.Login, owner, repo string) error {
 	}
 
 	// description
-	promptM := &survey.Multiline{Message: "Milestone description:"}
+	promptM := NewMultiline(Multiline{
+		Message:   "Milestone description:",
+		Syntax:    "md",
+		UseEditor: config.GetPreferences().Editor,
+	})
 	if err := survey.AskOne(promptM, &description); err != nil {
 		return err
 	}