add exactOptionalPropertyTypes support to html attributes

[deadem]

With exactOptionalPropertyTypes option enabled in tsconfig.json, assign undefined value to html attributes shouldn't generate an error.

const link: string | undefined
...
<a href={link}>Conditional link</a>

This code generates an error but should not.

React types declaration (for reference only): https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/index.d.ts#L1917

Preact playground

React playground

This PR continues #3711

[coveralls]

Coverage: 99.543%. Remained the same when pulling 1581fe0 on deadem:exactOptionalPropertyTypes into 8a30bcc on preactjs:master.

[andrewiggins]

To verify my understanding, with this new option turned on, TypeScript no longer "auto adds" undefined to the type of a property marked as optional using ?. Is that correct?

And this PR just adds undefined to all DOM properties so that we can pass undefined to them when this new option is turned on?

[deadem]

To verify my understanding, with this new option turned on, TypeScript no longer "auto adds" undefined to the type of a property marked as optional using ?. Is that correct?

Yes.

And this PR just adds undefined to all DOM properties so that we can pass undefined to them when this new option is turned on?

Yes.