feat mr: remove prefix in ID of MR just like Issue

This change

- improves consistency of APIs.
- is copied from: https://github.com/profclems/glab/blob/0232a0c0791374ef5c1fa2072582a7127126b078/commands/issue/issueutils/utils.go#L106
- makes pipes easier
    - e.g., `glab mr list | fzf | grep -oE "^\S+" | glab mr view`
    - this is what atusy/ghf internally does

Finishes #789

commands/mr/mrutils/mrutils.go

@@ -142,7 +142,7 @@ func MRFromArgsWithOpts(
 	var branch string
 
 	if len(args) > 0 {
-		mrID, err = strconv.Atoi(args[0])
+		mrID, err = strconv.Atoi(strings.TrimPrefix(args[0], "!"))
 		if err != nil {
 			branch = args[0]
 		} else if mrID == 0 { // to check for cases where the user explicitly specified mrID to be zero