Enforce dangling underscores in method names

[JBallin]

@ljharb updated the docs on 4/13/16 to forbid using underscores in method names, but the linter does not actually raise an error when defining a method with a dangling underscore.

// bad
this.__firstName__ = 'Panda';
this.firstName_ = 'Panda';
this._firstName = 'Panda';

// good
this.firstName = 'Panda';

@gabro added the ability to enforce underscores in method names to ESLint on 7/16/17. The rule was added to the linter's rules by @ljharb on 9/1/17, but was left as false instead of true, despite the TODO comment to 'enable' the rule (presumably this was human error).

Before most recent commit:

// enforceInMethodNames: false, // TODO: uncoment and enable, semver-minor once v3 is dropped

After most recent commit:

enforceInMethodNames: false,

After my commit:

enforceInMethodNames: true,

NOTE: The linter currently does complain when calling a method with a dangling underscore. This is addressing defining the method.

[ljharb]

Thanks! Indeed, this should have been enabled when v3 was dropped.