Update browser script to make changeable the target root

[yhatt]

Updated browser script to make changeable the target root. It brings that Marp slides can render correctly within the shadow root (Web Components).

The target root will change depending on the root node of an injected script automatically.

If using module from @marp-team/marp-core/browser, developer can change the target by passing the node in first argument.

import browser from '@marp-team/marp-core/browser`

// Apply to the shadow root
const container = document.querySelector('.marp-container')
const cleanup = browser(container.shadowRoot)

observe(), a detailed interface, has changed the form of argument to object. A previous boolean argument has deprecated and will be transformed to corresponding option if used.