Lint against export [some declaration]

This disallowd the following types of `export` declaration:
- `export class A {}`/`export function A() {}`
- `export default class A {}`/`export default function A() {}`
- `export let A`/`export const A`/`export var A`

While allowing
- `export { A }`
- `export default A`

src/pdf.sandbox.external.js

@@ -16,6 +16,7 @@
 // In mozilla-central, this file is loaded as non-module script,
 // so it mustn't have any dependencies.
 
+// eslint-disable-next-line no-restricted-syntax
 export class SandboxSupportBase {
   /**
    * @param {DOMWindow} - win