[fix] Better handling of inert attribute

[fcrozatier]

Currently the inert boolean attribute is not handled properly: like the disabled attribute it should not be set when false. But inspecting the DOM reveals it's set as inert="false" which is wrong for a boolean attribute

<script>
let disabled = false;
let inert = false;
</script>

<button {disabled}>click me</button>

<div {inert}>some div <button>click</button></div>

<style>
[inert] {
    opacity: 50%
}
</style>

I'm not too familiar with the code base so I just added the keyword to the attribute_lookup. Like the hidden attribute, inert can apply to any HTMLElement. Sorry if my fix is partial, let me know how to improve it.

Also mentioned here

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Tests

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[baseballyama]

Thank you for handling this!
I added tests and create a new PR.
#7944

[baseballyama]

Close in favor of #7944
I took your commit to #7944, so your name will list to commit log.