feat!: add no-loader-import-specifier rule (#122)

[wincent]

This rule prevents us from providing useless import specifiers when importing SCSS via the loader:

import useless from './styles.scss';

These modules should only be imported for their side-effects; ie:

import './styles.scss';

As noted in the docs, we might want to lint for other kinds of non-JS resources in the future, but for now ".scss" is the only file type that we actually have configured to load via this mechanism.

Test plan: Copy the code over to a liferay-portal repo (eg. with cp -R ~/code/eslint-config-liferay/{portal.js,plugins} node_modules/eslint-config-liferay) and then introduce a fake error; see this in the lint run output (yarn checkFormat):

modules/apps/frontend-js/frontend-js-components-web/src/main/resources/META-INF/resources/treeview/Treeview.js
  24:1  error  SCSS resources should be imported only for side-effects  liferay-portal/no-loader-import-specifier

Closes: #122

[jbalsas]

We could add .soy to the mix here... even if just for the sake of correctness and until we've gotten rid of them all...

[wincent]

We could add .soy to the mix here...

Ah, good idea!

[jbalsas]

Although now that I think of it, we might actually be using their return, so scratch that, I need coffee ☕️

We are loading some .js files just so they load .soy templates, but that'll be a whole different story

[wincent]

Common pattern:

import './ValidationRegister.soy.js';

Soy.register(Validation, templates);

and:

import './TextRegister.soy.js';