Add extension/ to .pubignore to reduce pub package size

[arrrrny]

Closes #427

The extension/ directory contains DevTools extension source that doesn't need to be published. Adding it to .pubignore will reduce the published package size.

[escamoteur]

Thanks for looking into the package size — it's a fair concern. But extension/devtools/build/ is not source, it's the compiled DevTools extension that pub.dev has to ship: DevTools loads it from exactly that path in the published package (see devtools: extension: in pubspec.yaml; .pubignore already has !extension/devtools/build/ on purpose, and the actual source under tool/get_it_devtools_extension/ is already excluded). Adding extension/ would silently break the DevTools extension for every user.

What we did instead for 9.3.0: dropped the *.js.symbols files and .last_build_id from the published build (~9 MB uncompressed). Loading CanvasKit from the CDN instead of bundling it is on the list as a follow-up.