Added linting support for private class methods

[giovannicalo]

Q	A
Fixed Issues?	No
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	Yes
Documentation PR Link	No
Any Dependency Changes?	No
License	MIT

The ESLint plugin claims its @babel/no-invalid-this rule handles private class methods correctly, however that doesn't seem to be the case.

Running

eslint -f pretty a.js

on

class A {

    #a() {
        return this.b;
    }

}

with ESLint settings

module.exports = {
    parser: "@babel/eslint-parser",
    plugins: ["@babel"],
    rules: { "@babel/no-invalid-this": 2 }
};

and Babel settings

module.exports = {
    presets: [["@babel/env", {
        shippedProposals: true
    }]]
};

prints

  a.js:4:10
  ×  4:10  Unexpected this.  @babel/no-invalid-this

  1 error

even though that this is perfectly valid.

This fixes the issue.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/27779/

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[kaicataldo]

One suggestion for a clearer variable name, but otherwise LGTM!