Make sure polyfills.js is imported before anything else

[rsimha]

In #13256, we introduced new eslint rules to keep our import and require statements sorted. However, some modules like polyfill.js must be imported before any others.

This PR adds eslint rule exceptions to all import './polyfills'; statements to ensure that they appear at the top of their respective files. This will prevent gulp lint from complaining if another file-level import that alphabetically precedes ./polyfill (for example, ./foo) is added below it.

Will generate a linter error:

// src/polyfills.js must be the first import.
import './polyfills'; // eslint-disable-line sort-imports-es6-autofix/sort-imports-es6
import './foo';

Won't generate a linter error:

// src/polyfills.js must be the first import.
import './polyfills'; // eslint-disable-line sort-imports-es6-autofix/sort-imports-es6
import './foo'; // eslint-disable-line sort-imports-es6-autofix/sort-imports-es6

[rsimha]

/to @jridgewell @erwinmombay

Are there any other imports like polyfills.js that we'd like to ensure stay at the top?