Feat: add loading dom prop

[LukasDeco]

I would like to add the loading property for Domprops. This allows someone to add the loading property on an image for using html-based browser-native lazy loading.

[yawaramin]

We can specify the values exactly at the type level:

loading: [#lazy | #eager],

[LukasDeco]

Okay I made the change. I had to wrap lazy in quotes because it is a reserved keyword. Also, I used string because I noticed other props that are have a small/finite number of options just use string. (Like method and inputMode)

[Kingdutch]

This PR fixes #43 once the feedback from yawaramin is fixed :)

[yawaramin]

Thanks. The other props use string type because they predate ReScript's feature of polymorphic variants compiling to strings. I think it would make sense to update the prop types where appropriate.

[LukasDeco]

Is that something we want to do on this PR?

[yawaramin]

Not necessarily. I +1'd this PR as I don't think it needs to do anything else. My comment was more about future improvements.

[LukasDeco]

@Kingdutch @yawaramin sorry for the delay. I can't seem to merge this PR though.

[mununki]

Hi, I'm quite late for the party.
I'm wondering why the loading attribute is added into type domProps only, not into type props.