fix: contextual_help uses heading, content, footer props

[wusteven815]

• Fixes ui.contextual_help content not rendering properly #926
• Auto-wraps heading, content, and footer props in their respective component

BREAKING CHANGE: ui.contextual_help's signature replaces the *children prop with heading, content, and footer props, in that order. In parity with React Spectrum docs, the heading and content props are required and footer is optional.

Here are some examples of usages and how they might change:

# This is still valid
old = ui.contextual_help(
    ui.heading("My Heading"),
    ui.content("My content"),
    ui.footer("My footer")
)

# This is invalid
old = ui.contextual_help(ui.content("My content"))
# You must explicitly set heading to None
new = ui.contextual_help(None, ui.content("My content"))