Support "Endless method definition" syntax for Ruby 2.8 (3.0)

This PR supports "Endless method definition" syntax for Ruby 2.8 (3.0).
Parser gem supports this syntax by whitequark/parser#676.

Ref: https://bugs.ruby-lang.org/issues/16746

CHANGELOG.md

@@ -13,6 +13,7 @@
 * [#41](https://github.com/rubocop-hq/rubocop-ast/pull/41): Add `delimiters` and related predicates for `RegexpNode`. ([@owst][])
 * [#46](https://github.com/rubocop-hq/rubocop-ast/pull/46): Basic support for [non-legacy AST output from parser](https://github.com/whitequark/parser/#usage). Note that there is no support (yet) in main RuboCop gem. ([@marcandre][])
 * [#48](https://github.com/rubocop-hq/rubocop-ast/pull/48): Support `Parser::Ruby28` for Ruby 2.8 (3.0) parser. ([@koic][])
+* [#49](https://github.com/rubocop-hq/rubocop-ast/pull/49): Support "Endless method definition" syntax for Ruby 2.8 (3.0). ([@koic][])
 
 ## 0.0.3 (2020-05-15)
 

lib/rubocop/ast/builder.rb

@@ -26,6 +26,7 @@ class Builder < Parser::Builders::Default
         case:         CaseNode,
         class:        ClassNode,
         def:          DefNode,
+        def_e:        DefNode,
         defined?:     DefinedNode,
         defs:         DefNode,
         ensure:       EnsureNode,

spec/rubocop/ast/def_node_spec.rb

@@ -15,6 +15,12 @@
 
       it { expect(def_node.is_a?(described_class)).to be(true) }
     end
+
+    context 'with a def_e node', :ruby28 do
+      let(:source) { 'def foo() = 42' }
+
+      it { expect(def_node.is_a?(described_class)).to be(true) }
+    end
   end
 
   describe '#method_name' do
@@ -41,6 +47,12 @@
 
       it { expect(def_node.method_name).to eq(:-@) }
     end
+
+    context 'with endless method definition', :ruby28 do
+      let(:source) { 'def foo() = 42' }
+
+      it { expect(def_node.method_name).to eq(:foo) }
+    end
   end
 
   describe '#method?' do
@@ -109,6 +121,12 @@
 
       it { expect(def_node.arguments.size).to eq(1) }
     end
+
+    context 'with endless method definition', :ruby28 do
+      let(:source) { 'def foo(bar, baz) = 42' }
+
+      it { expect(def_node.arguments.size).to eq(2) }
+    end
   end
 
   describe '#first_argument' do

spec/spec_helper.rb

@@ -30,6 +30,10 @@
   let(:ruby_version) { 2.7 }
 end
 
+RSpec.shared_context 'ruby 2.8', :ruby28 do
+  let(:ruby_version) { 2.8 }
+end
+
 module DefaultRubyVersion
   extend RSpec::SharedContext