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