feat!: enforce use of strict (===) equality

[wincent]

ESLint can enforce this for us with the built-in "eqeqeq" rule, which you might not know about if you'd search for "strict", "equality" or "equal" in the rules list.

Idea is to avoid coercion due to ==/!= which can be a source of bugs, except in the case of null, where x == null is a convenient and idiomatic shorthand for x === null || x === undefined (you can still do an explicit x === null check if that's what you want).

Docs: https://eslint.org/docs/rules/eqeqeq

Closes: https://github.com/liferay/eslint-config-liferay/issues/158

[wincent]

FWIW, tested this in liferay-portal with:

cp ~/code/eslint-config-liferay/index.js ~modules/node_modules/eslint-config-liferay
portool yarn run liferay-npm-scripts lint  

Finds a bunch of errors:

✖ 364 problems (364 errors, 0 warnings)
  21 errors and 0 warnings potentially fixable with the `--fix` option.

Autofix diff is (14 files changed, 24 insertions(+), 21 deletions(-)):

https://gist.github.com/wincent/9b1925268aa56312106908b892c09138

Not-autofixable stuff is like this:

		return pauseNodes.filter(({id}) =>
			nodeKeys.find(node => node.id == id)
		);

All of these require human review to be "sure" they're safe, and even then I don't think we can be sure. I think we just have to ship it, run the tests, and watch for regressions.