feat(runtime): allow mutation observer optimization

[exside]

This is probably a more advanced usage config, but as it doesn't affect default behaviour I think it's a good addition with minimal bundle size impact.

The runtime's MutationObserver is basically watching the entire DOM for all mutations on all nodes and attributes, that's probably fine for most, but I would prefer if I could fine tune Uno CSS runtime so it does the least amount of work necessary, therefore I propose an addition to the runtime options, which allow granular control over:

a) what area of the DOM is monitored by returning a node to watch (including its child nodes) with target
b) which attributes to watch for mutations by providing an attributeFilter array

both config options are of course optional, default behaviour stays exactly as it is now and are summarized under observer

So now we could do this:

		...,
		runtime: {
			observer: {
				target: () => document.querySelector('.your-area-of-the-site-to-watch-for-mutations'),
				attributeFilter: ['class'], // only watch for changes in class attribute not all attributes
			},
		},
		...

the attributeFilter option for observe can only be included if defined, we can't just specify an empty array [] as default, then the observer will watch no attribute at all, therefore this slightly ugly construct:

...(runtimeOptions.observer?.attributeFilter ? { attributeFilter: runtimeOptions.observer.attributeFilter } : {}),

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[chu121su12]

Maybe we don't need target() as function?

Also please add the new prop types definition

[exside]

removed the target() as a function, now a regular const

Tried to add the type defs, hope it's ok 😬

[ydcjeff]

Generally LTGM.
Need to wait antfu for final review.