feature: add a new commit message trailer - Milestone:

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/Views/CommitMessageToolBox.axaml.cs

@@ -385,6 +385,7 @@ private void OnCaretPositionChanged(object sender, EventArgs e)
             "Fixes: ",
             "Helped-by: ",
             "Issue: ",
+            "Milestone: ",
             "on-behalf-of: @",
             "Reference-to: ",
             "Refs: ",