Add element.onEndTag js binding

[taina0407]

This PR is to support onEndTag for JS API.
This is the first time I'm working on a Rust project so feel free to suggest any changes

[kornelski]

There's only one EndTag type, so the macro for defining mutation methods seems unnecessary?

[taina0407]

There's only one EndTag type, so the macro for defining mutation methods seems unnecessary?

You are right. The problem here is EndTag seems missing removed property, so I changed the PR to adding this so we can re-use existing impl_mutations macro instead.

[kornelski]

Looks good. Small nit: the comment for removed is a copypaste error

[taina0407]

@kornelski thankyou, amend the commit to change "comment" to "end tag" :)