TS: Add missing types: is***

[yomotsu]

isXXX is missing in d.ts.
This PR is to fill them.

[Mugen87]

Um, can we mark all is* properties as read-only?

Related #18217 .

[yomotsu]

Thanks for the suggestion.
Regarding it, Which one is preferable?

• readonly isBox3: boolean
• isBox3: true

Seems some are already isXXX: true like
https://github.com/mrdoob/three.js/blob/dev/src/math/Vector3.d.ts#L30

But we can respect the actual value in .js file (which is true) to set boolean as readonly as you suggested.

or maybe both: readonly isBox3: true

[donmccurdy]

I'd suggest both: readonly isBox3: true 👍

[yomotsu]

Done it 👍

[mrdoob]

Thanks!