[java] New Rule: Always compare enum values with `==`

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Proposed Rule Name: ❓ UnnecessaryEqualsForEnum | EnumComparison

Proposed Category: ❓ Best Practices | Code Style

Description:
See #6116 (comment)

Enum should be compared using ==, this is more readable. Using equals() is not necessary (and is prone to NPE).

Code Sample:

class Foo {
  void method(Thread.State state1, Thread.State state2) {
    if (state1.equals(state2)) { }  // violation

    if (state1 == state2) {}  // correct
  }
}

Possible XPath expression:

//MethodCall[pmd-java:matchesSig("_#equals(java.lang.Object)")]
  [VariableAccess[pmd-java:typeIs("java.lang.Enum")]]
  [ArgumentList[VariableAccess[pmd-java:typeIs("java.lang.Enum")]]]

Possible Properties:

• none

References:

• https://sonarsource.github.io/rspec/#/rspec/S4551
• https://www.infoworld.com/article/2163724/use-or-to-compare-java-enums.html
• https://stackoverflow.com/questions/1750435/comparing-java-enum-members-or-equals