Rename output modes

[jayconrod]

(Migrated from bazelbuild/rules_go#693)

Gazelle -mode=fix is confusingly similar to the recently added fix command. I'd like to rename -mode to -output_mode and -mode=fix to -output_mode=write.

To ease the transition, -mode and -mode=fix will continue to work as aliases for the new names, printing warnings when used. The old names can be removed in a few months.