[gql_build] generate aliases for the generated data, reqs and vars

[osaxma]

Since not too long ago, dart introduced Typedefs for non-function types*. It'd be nice if gql_build can generate an optional aliases file such as:

• generated file: ./all_pokemon.aliases.dart;

import './all_pokemon.req.gql.dart';
import './all_pokemon.var.gql.dart';
import './all_pokemon.data.gql.dart';

typedef AllPokemon = GAllPokemon;
typedef AllPokemonData = GAllPokemonData;
typedef AllPokemonVar = GAllPokemonVars;
typedef AllPokemonVarsBuilder = GAllPokemonVarsBuilder;

• In some other file:

import './all_pokemon.aliases.dart';

final pokemonQuery = AllPokemon((b) {
  b.vars = AllPokemonVarsBuilder()..first = 100;
});

final allPokemonsData = AllPokemonData.fromJson({/* json */});

The main advantage will be:

• IDE can support Autocomplete since all_pokemon.aliases.dart can be included in the analyzer.
• The G prefix can be removed by the alias.
• The alias file is browsable for available data, reqs, and vars.

* Typedefs for non-function types this is only supported if the min sdk constraint is set to 2.15.0 or higher in pubspec.yaml