Update react-theme-provider to v2

[hawkrives]

Closes #3549

---

Converts us from ThemeProvider and the withTheme higher-order component (HOC) to ThemeProvider and typed(!) useTheme hook.

Old

type HocProps = {
	theme: AppTheme,
}

type BaseProps = {
	size: number,
}

type Props = BaseProps & HocProps

function Box({theme, size}: Props) {
 	return <View style={{width: size, height: size, backgroundColor: theme.accent}} />
}

const ThemedDottedBar: React.StatelessFunctionalComponent<BaseProps> = (withTheme(
	DottedBar,
): any)

New

type Props = {
	size: number,
}

function Box({size}: Props) {
	let theme = useTheme()
	return <View style={{width: size, height: size, backgroundColor: theme.accent}} />
}

and useTheme automatically(*) knows the shape of our theme.

*: it's defined in @frogpond/theme

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NOTE: I don't think I've tried running this locally, but it was blocked on react-theme-provider@2.0.1, which just came out, so I wanted to get this out.

This also has an alternate version of useViewport from #3613; I will resolve the conflict with whichever one goes in first. They're roughly equivalent.