feat: better deephaven ui button defaults

[dsmmcken]

Make accent the default variant, and fill the default style.

1. Buttons should have a default variant and style specified., since variant is actually a required prop. Style being set to fill makes using static_color easier.
2. Remove deprecated cta and overBackground variants.
3. element_type wasn't plumbed through but needs to be set to a for href link buttons to work. I've made this internal and just auto set it for users if href is set.

BREAKING CHANGE:

ui.button defaults changed to variant="accent" and style="fill". To restore previous styling, use variant="primary" and style="outline".

[dsmmcken]

No idea where you got cta and overBackground, co-pilot hallucinations?

I see now they are deprecated spectrum legacy options:

/** @deprecated */
type LegacyButtonVariant = 'cta' | 'overBackground';