Fix exception when field name contains all invalid characters

[aaronrenner]

This exception is raised when the entire artifact name is invalid (like "1").

== Compilation error in file lib/my_app/schema.ex ==
** (MatchError) no match of right hand side value: nil
    lib/absinthe/phase/schema/validation/names_must_be_valid.ex:34: Absinthe.Phase.Schema.Validation.NamesMustBeValid.valid_name?/1
    lib/absinthe/phase/schema/validation/names_must_be_valid.ex:20: Absinthe.Phase.Schema.Validation.NamesMustBeValid.validate_names/1
    lib/absinthe/blueprint/transform.ex:16: anonymous fn/3 in Absinthe.Blueprint.Transform.prewalk/2
    lib/absinthe/blueprint/transform.ex:109: Absinthe.Blueprint.Transform.walk/4
    (elixir 1.10.2) lib/enum.ex:1520: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    lib/absinthe/blueprint/transform.ex:145: anonymous fn/4 in Absinthe.Blueprint.Transform.walk_children/5
    (elixir 1.10.2) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
    lib/absinthe/blueprint/transform.ex:114: Absinthe.Blueprint.Transform.walk/4
    (elixir 1.10.2) lib/enum.ex:1520: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    (elixir 1.10.2) lib/enum.ex:1520: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    lib/absinthe/blueprint/transform.ex:145: anonymous fn/4 in Absinthe.Blueprint.Transform.walk_children/5
    (elixir 1.10.2) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
    lib/absinthe/blueprint/transform.ex:114: Absinthe.Blueprint.Transform.walk/4
    (elixir 1.10.2) lib/enum.ex:1520: Enum."-map_reduce/3-lists^mapfoldl/2-0-"/3
    lib/absinthe/blueprint/transform.ex:145: anonymous fn/4 in Absinthe.Blueprint.Transform.walk_children/5
    (elixir 1.10.2) lib/enum.ex:2111: Enum."-reduce/3-lists^foldl/2-0-"/3
    lib/absinthe/blueprint/transform.ex:114: Absinthe.Blueprint.Transform.walk/4
    lib/absinthe/blueprint/transform.ex:15: Absinthe.Blueprint.Transform.prewalk/2
    lib/absinthe/phase/schema/validation/names_must_be_valid.ex:11: Absinthe.Phase.Schema.Validation.NamesMustBeValid.run/2
    lib/absinthe/pipeline.ex:369: Absinthe.Pipeline.run_phase/3

This fixes that exception by simplifying the code by using Regex.match?/2 together with beginning and end of line anchors.

It also adds a new kind: "enum value".

[benwilson512]

Thanks!