updateStyle(): use setProperty() when css vars and dashed-properties, fixes #2989

[kfule]

Description

This PR changes updateStyle() to use setProperty() for dashed-properties.

Motivation and Context

This PR maybe fixes #2989

Current mithril (v2.2.10) doesn't seem to work well when using something like domMock, which doesn't have enough support for dashed-properties.
It may not be a browser that Mithril officially supports. But the fix is clear and has little or no impact on performance as discussed in #2985.

How Has This Been Tested?

npm run test

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]

Welp, thought we could get away with #2985 (comment), but turns out that was too breaking...

Edit: I'm keeping it in #2982, though - anyone claiming to implement a web browser or the full DOM should be implementing those properties, and bug reports should be directed at those who claim to do so and don't.

[dead-claudia]

Please add a test spying on a style setter to make sure we're only using camel case setters.

[kfule]

@dead-claudia OK, test added.