Modifies the update_dev_manifest script

[elliette]

• Changes the update_dev_manifest.dart script (added in Update builder logic for the Dart Debug Extension #1717) to update_dev_files.dart script

Previously, the script:

• Added the Googler-only extension key (in a gitignored txt file) to the manifest.json (in the gitignored dev_build directory)
• Changed the manifest.json icon from the normal icon to the dev icon

With this change, the script also:

• Adds "[DEV]" to the start of the extension name to make it easy to distinguish from the non-dev version
• Changes background.js to be background.dart.js for DDC
• Changes the devtool.js file to prefix the Dart Debugger and Flutter Inspector panel names in Chrome DevTools with "[Dev]"