Update gulp-eslint version to 4.0

[rsimha]

This PR does the following:

1. Updates the version of gulp-eslint used by gulp lint to 4.0
2. Removes the deprecated ecmaFeatures section of .eslintrc (we've already set ecmaVersion to 6) (See https://eslint.org/docs/user-guide/migrating-to-2.0.0#language-options)
3. Fixes several indentation errors detected by a more strict indent rule (See https://eslint.org/docs/user-guide/migrating-to-4.0.0#indent-rewrite)
4. Fixes several comment spacing errors detected by a more strict no-multi-spaces rule (See https://eslint.org/docs/user-guide/migrating-to-4.0.0#-the-no-multi-spaces-rule-is-more-strict-by-default)

Partial fix for #12181
Follow up to #12350

[rsimha]

/to @aghassemi @choumx @erwinmombay

[rsimha]

For a more sane code review without whitespace-only changes, use this link: https://github.com/ampproject/amphtml/pull/12450/files?w=1

[aghassemi]

My only comment is about making sure that gulp lint still picks up custom rules defined in elint-rules folder, my worry is that maybe gulp-eslint v4 has followed the path of eslint v4 and removed support for rulesPath in favour of either rulesdir or plugins.

One way to verify is using the banned spread operator. In any file change a function call like foo(bar); to foo(...bar); and run gulp lint if it does not complain, custom rules are not being picked up.

[rsimha]

@aghassemi The rules in eslint-rules do get picked up. I've verified this by running

DEBUG=eslint:cli-engine gulp lint

Output:

~/src/amphtml$ DEBUG=eslint:cli-engine gulp lint
[18:49:25] Using gulpfile ~/src/amphtml/gulpfile.js
[18:49:25] Starting 'lint'...
  eslint:cli-engine Loading rules from build-system/eslint-rules/ +0ms
  eslint:cli-engine Linting /usr/local/google/home/rsimha/src/amphtml/3p/3p.js +752ms
  eslint:cli-engine Linting /usr/local/google/home/rsimha/src/amphtml/3p/ampcontext-integration.js +1s
  eslint:cli-engine Linting /usr/local/google/home/rsimha/src/amphtml/3p/ampcontext-lib.js +458ms
  ...
  eslint:cli-engine Linting /usr/local/google/home/rsimha/src/amphtml/extensions/amp-viewer-integration/0.1/test/integration/test-amp-viewer-integration.js +36ms
  eslint:cli-engine Linting /usr/local/google/home/rsimha/src/amphtml/extensions/amp-viewer-integration/0.1/test/integration/test-amp-webview-viewer-integration.js +78ms
  eslint:cli-engine Linting /usr/local/google/home/rsimha/src/amphtml/extensions/amp-viewer-integration/0.1/test/integration/test-viewer-initiated-handshake.js +18ms
[18:51:09] Finished 'lint' after 39 s

[rsimha]

Double-verified by making sure that adding a spread operator will result in a lint error :)

[aghassemi]

Thanks @rsimha-amp . LGTM.