Enable and fix package:pedantic lints (dart-lang/stack_trace#62)

- always_declare_return_types
- annotate_overrides
- prefer_conditional_assignment
- prefer_if_null_operators
- prefer_single_quotes
- unnecessary_this
- use_function_type_syntax_for_parameters

Remove local lint config which duplicates enabled rules.

Ignore `prefer_spread_collections` for now. We should publish once more
before bumping the min SDK.