feat: add pull_description to placeholders

This allows the user to copy the original pull request description
to the backported PR. This can be helpful if the body contains
links to ticketing systems, related documents etc.

Example usage:

yaml
```
          pull_description: |-
            Automated backport to `${target_branch}`, triggered by a
label in #${pull_number}.

            Original Description:
            ${pull_description}
```

README.md

@@ -243,6 +243,7 @@ Placeholder | Replaced with
 `pull_number` | The number of the original pull request that is backported, e.g. `123`
 `pull_title` | The title of the original pull request that is backported, e.g. `fix: some error`
 `target_branch`| The branchname to which the pull request is backported, e.g. `release-0.23`
+`pull_description`| The original description of the pull request you created the backport from, e.g. `Summary: This patch was created to..`
 
 ## Outputs
 

dist/index.js

@@ -990,11 +990,13 @@ exports.getMentionedIssueRefs = exports.replacePlaceholders = void 0;
  * @returns Description that can be used in the backport pull request
  */
 function replacePlaceholders(template, main, target) {
+    var _a;
     const issues = getMentionedIssueRefs(main.body);
     return template
         .replace("${pull_author}", main.user.login)
         .replace("${pull_number}", main.number.toString())
         .replace("${pull_title}", main.title)
+        .replace("${pull_description", (_a = main.body) !== null && _a !== void 0 ? _a : "")
         .replace("${target_branch}", target)
         .replace("${issue_refs}", issues.join(" "));
 }

src/utils.ts

@@ -16,6 +16,7 @@ export function replacePlaceholders(
     .replace("${pull_author}", main.user.login)
     .replace("${pull_number}", main.number.toString())
     .replace("${pull_title}", main.title)
+    .replace("${pull_description", main.body ?? "")
     .replace("${target_branch}", target)
     .replace("${issue_refs}", issues.join(" "));
 }