fix(watch): deep watching symbol properties

[skirtles-code]

Currently, deep watchers don't traverse symbol properties.

#402 already attempts a fix for this, but that seems to have stalled due to some performance tweaks that were made at the same time. Here I've just tried to fix the core problem with symbols.

String properties are traversed using for/in. That traverses enumerable properties, including those inherited from the prototype. I didn't want to do anything that might break that, so I've left it untouched.

For symbols I've used getOwnPropertySymbols and propertyIsEnumerable. This doesn't take the prototype chain into account.

Is traversing symbol properties actually useful? I don't know. If not, perhaps it should be documented that they aren't supported? Currently it's a bit inconsistent, as adding or removing a symbol property will trigger the watcher, but updating the value or mutating a nested value won't.

I've marked this as a fix, though it could be viewed as a feat.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	90.8 kB (+110 B)	34.5 kB (+35 B)	31.2 kB (+99 B)
vue.global.prod.js	148 kB (+110 B)	53.7 kB (+26 B)	48 kB (+1 B)

Usages

Name	Size	Gzip	Brotli
createApp	50.9 kB (+110 B)	19.9 kB (+32 B)	18.1 kB (+36 B)
createSSRApp	54.2 kB (+110 B)	21.2 kB (+30 B)	19.3 kB (+35 B)
defineCustomElement	53.2 kB (+110 B)	20.6 kB (+33 B)	18.8 kB (+36 B)
overall	64.6 kB (+110 B)	24.9 kB (+31 B)	22.6 kB (-12 B)