Allow auto-detection of imported enum values

[Amphaal]

Motivation Since the introduction of enhanced enums in Dart 2.17 (https://dart.dev/language/enums#declaring-enhanced-enums), associating metadata and custom behavior to enums is a common pattern, hence dicouraging using generator tools to generate enums. As Slang allows translations of existing enums via imports / generate_enum, we still need to explicitly list again all enums values in contexts.[existing_enum].enum, which naively seems redundant in cases where we need all enums values to be translated (which, IMO, represents the majority of cases).

Developer Experience Could we consider, instead of:

# Config
imports:
  - 'package:my_package/path_to_enum.dart' # define where your enum is
contexts:
  UserType:
    enum:
      - user
      - admin
    generate_enum: false # turn off enum generation 

... just do:

# Config
imports:
  - 'package:my_package/path_to_enum.dart' # define where your enum is
contexts:
  UserType:
    generate_enum: false # turn off enum generation

... and let slang search for UserType in imported dart files + check for presence of all expected translations based oncontexts.[existing_enum].paths / auto-detected paths ?

[Tienisto]

Parsing the source code will be difficult as slang does not know where the file is located (it may be in another package), but slang could infer the enum values from the translation files

[Tienisto]

Since 3.19.0, you can leave out the enum values.