Refactor `JsonTreeStyle`

[dmackdev]

Some small refactors to improve the JsonTreeStyle customisation API.

• Move abbreviate_root into JsonTreeStyle,
• Move toggle_buttons_state into JsonTreeStyle,
• Add builder methods to JsonTreeStyle for more fluent construction.

I think default_expand can remain where it is (set by a builder method on JsonTree) since it pertains to the behaviour of the tree rather than its appearance.