DevTools: Implement support for showing source_content in Debugger > Source panel (#36774)

This patch adds support for showing source_content in `Debugger >
Source` panel. This works by handling the clients `source` messages in
the source actor. These source actors are already advertised as resource
via the watcher, populating the source list. We also update the
`sources` handler in thread actor for future work in thread debugging.

Note: while this PR also adds support for showing worker script
source_content, worker has been broken (See
https://github.com/servo/servo/issues/37012). I was able to confirm the
`content_type` and `source_content` for worker script in logs.


![image](https://github.com/user-attachments/assets/bd53ea29-003a-4b5e-a3e8-6e280afa4671)

Fixes: part of https://github.com/servo/servo/issues/36027

---------

Signed-off-by: atbrakhi <atbrakhi@igalia.com>

components/shared/devtools/lib.rs

@@ -563,4 +563,6 @@ pub struct SourceInfo {
     pub url: ServoUrl,
     pub external: bool,
     pub worker_id: Option<WorkerId>,
+    pub content: String,
+    pub content_type: Option<String>,
 }