fix(reactivity): ensure readonly on plain arrays doesn't track array methods. (close: #2493)

[LinusBorg]

When using readonly() on a plain array, accessing the array in any way should not result in any tracking, no deps should be collected, as a plain array can't be tracked anyway.

However, for array methods like includes, we currently use arrayImplementations that do track wether or not the proxy is a reactive or readonly one.

Consequently, this doesn't result in tracking:

readonly([1, 2])[0]

but this does:

readonly([1, 2]).includes(1)

close: #2493

[github-actions]

Size report

Path	Size
vue.global.prod.js	40.15 KB (-0.01% 🔽)
runtime-dom.global.prod.js	26.43 KB (-0.01% 🔽)
size-check.global.prod.js	15.95 KB (-0.01% 🔽)