Add pull requests to git blame tooltip (#10784)

Release Notes:

- Added links to GitHub pull requests to the git blame tooltips, if they
are available.

Screenshot:

(Yes, the icon will be resized! cc @iamnbutler)

![screenshot-2024-04-19-18 31
13@2x](https://github.com/zed-industries/zed/assets/1185253/774af0b3-f587-4acc-aa1e-1846c2bec127)

crates/ui/src/components/icon.rs

@@ -121,6 +121,7 @@ pub enum IconName {
     WholeWord,
     XCircle,
     ZedXCopilot,
+    PullRequest,
 }
 
 impl IconName {
@@ -222,6 +223,7 @@ impl IconName {
             IconName::WholeWord => "icons/word_search.svg",
             IconName::XCircle => "icons/error.svg",
             IconName::ZedXCopilot => "icons/zed_x_copilot.svg",
+            IconName::PullRequest => "icons/pull_request.svg",
         }
     }
 }