Add support for custom states

[KonnorRogers]

Fixes #246

I went with @customState instead of @state to disambiguate from @state() decorator from Lit.

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[thepassle]

should we also analyze for this.internals.states.add("foo")?

During class analysis we can see if this.attachInternals() is assigned to something, keep that something in memory, and then see if <whatever>.states.add("foo") is being called with a string ("foo") on it?

[vospascal]

Fixes #246

I went with @customState instead of @state to disambiguate from @state() decorator from Lit.

this for me feels correct but also not attached to a name as long as its specific enough if your looking bit in to the future there going to be more exposed things other then parts and these customState/cssState might be hard to keep these consistant like cross-root-aria proposal if the exportids is uses then you would get ids :)

[KonnorRogers]

should we also analyze for this.internals.states.add("foo")?

During class analysis we can see if this.attachInternals() is assigned to something, keep that something in memory, and then see if <whatever>.states.add("foo") is being called with a string ("foo") on it?

Sounds similar to how events are "discovered", so I don't see why not

[thepassle]

@KonnorRogers https://github.com/webcomponents/custom-elements-manifest/pull/128 was merged, can you update this PR? :)

[KonnorRogers]

@thepassle updated to use @cssstate and @cssState pushing to cssStates array.

I also updated the README to reflect the new supported tag.

I also added a new fixture directory for it.

[thepassle]

Thanks for the PR! Published in 0.10.0