feat: Make redraws when Promises returned by event handlers are completed

[kfule]

This PR allows redraw on completion of the async event handler.

Description

This PR makes redraws when Promises returned by event handlers are completed. This allows for async event handlers to be redrawn on completion of the function as well as synchronous event handlers.

Motivation and Context

This is like re-opening the #2862 that was accidentally closed.
Unlike #2862 , this PR uses Promise.finally to handle the return value and also adds the condition event.redraw === false. Therefore, I did not cherry-pick #2862 .

How Has This Been Tested?

I added the tests and confirmed that npm run test passes.
I also added eslintrc because the new test code includes the async functions.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My change requires a documentation update, and I've opened a pull request to update it already:
• I have read https://mithril.js.org/contributing.html.

[dead-claudia]

I like the basic idea, but not the current execution of it.

Doing redraws always for promises and only on success for normal functions will be too surprising to people when errors are involved.

[dead-claudia]

Suggested change

	Promise.resolve(result).finally(function () {
	Promise.resolve(result).then(function () {

I'd rather rejections and thrown errors have the same behavior. It's less surprising to users.

Obviously, this will also require changes to the tests.

[kfule]

OK, I changed finally to then and fixed the test.

[JAForbes]

Note: @dead-claudia @kfule I don't think this should be a semver major change. I've started a thread on zulip here for us to discuss.

[dead-claudia]

@JAForbes And responded. We can continue the discussion there.