Support setting milestone during issue creation

Sadly this is not supported when creating a Github pull-request.
Gitlab support will follow later.

Author: tarsius

lisp/forge-github.el

@@ -734,6 +734,12 @@
     (pcase-let ((`(,title . ,body) (forge--post-buffer-text)))
       `((title . ,title)
         (body  . ,body)
+        ,@(and forge--buffer-milestone
+               `((milestone
+                  . ,(caar (forge-sql [:select [number]
+                                       :from milestone
+                                       :where (= title $s1)]
+                                      forge--buffer-milestone)))))
         ,@(and forge--buffer-labels
                `((labels . ,(vconcat forge--buffer-labels))))
         ,@(and forge--buffer-assignees

lisp/forge-post.el

@@ -112,6 +112,7 @@ an error."
 (defvar-local forge--buffer-post-object nil)
 (defvar-local forge--buffer-template nil)
 (defvar-local forge--buffer-category nil)
+(defvar-local forge--buffer-milestone nil)
 (defvar-local forge--buffer-labels nil)
 (defvar-local forge--buffer-assignees nil)
 (defvar-local forge--buffer-base-branch nil)
@@ -249,6 +250,7 @@ an error."
           (and (forge-github-repository-p (forge-get-repository :tracked))
                (string-prefix-p "new-"
                                 (file-name-nondirectory buffer-file-name))))
+    ("-m" forge-new-topic-set-milestone)
     ("-l" forge-new-topic-set-labels)
     ("-a" forge-new-topic-set-assignees)
     ("-d" forge-new-pullreq-toggle-draft)]
@@ -286,6 +288,15 @@ an error."
                                   (format "%s %s" name value)
                                 (format "%s" name)))))))
 
+(transient-define-infix forge-new-topic-set-milestone ()
+  "Set milestone for the topic being created."
+  :class 'forge--new-topic-set-slot-command
+  :variable 'forge--buffer-milestone
+  :name "milestone"
+  :reader (lambda (&rest _) (forge-read-topic-milestone))
+  :formatter (lambda (milestone) (propertize milestone 'face 'forge-topic-label))
+  :if (lambda () (equal (file-name-nondirectory buffer-file-name) "new-issue")))
+
 (transient-define-infix forge-new-topic-set-labels ()
   "Set labels for the topic being created."
   :class 'forge--new-topic-set-slot-command