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.

Fixes: part of https://github.com/servo/servo/issues/36027
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Signed-off-by: atbrakhi <atbrakhi@igalia.com>
Currently Source related code exists in watcher.rs and thread.rs. This
change moves source-related code to a dedicated source.rs file. This is
in preparation for adding support for showing source code in the
Debugger > Source panel.
- [x] Testing: These changes should not affect current functionality as
it only moves the existing code
- [x] Fixes: part of https://github.com/servo/servo/issues/36027
---------
Signed-off-by: atbrakhi <atbrakhi@igalia.com>