Format all source files with buildifier

[bartekpacia]

I also updated GH workflows and added enforcing formatting to CI.

Apart from that, I just run buildifier -r . in root directory.

[SpencerC]

Oops just saw this! Added #91 and #92 that do the same thing. @cbracken looks like the choice is yours. 😝

[bartekpacia]

Hahah, nice to see someone else being interested in Bazel for Dart! :D