feat (no-invalid-extends): new rule

[43081j]

Enforces that custom elements extend from a list of allowed classes.

By default, the only allowed class is HTMLElement unless you use the extends option when defining the element. In those cases, it must extend the known built-in class (e.g. HTMLDivElement).

For example:

// invalid

/** @customElement */
class Foo extends Bar {}
customElements.define('foo', Foo);

// valid

/** @customElement */
class Foo extends HTMLElement {}
customElements.define('foo', Foo);

// also valid

/** @customElement */
class Foo extends HTMLDivElement {}
customElements.define('foo', Foo, {extends: 'div'});

// also invalid

/** @customElement */
class Foo extends HTMLDivElement {}
customElements.define('foo', Foo);

cc @keithamus

[keithamus]

Looks real nice!

[43081j]

also dropped node 12 from CI since this moves to outputting es2021 (i.e. optional chaining)

think we should be ok, though may break some old school consumers but at least they can stay on the old version