Create a helper `isPWA(dom)`

[sarvaje]

Not blocking this PR, but since we know we're going to use this across a few hints, what do you think about making an isPWA(dom) helper method to share between them? In theory this would also make changing our "detection" logic easier in the future if needed.

Originally posted by @antross in #3301

[molant]

@sarvaje can you please list the hints that will need to be updated to use this new feature in case someone with less background wants to take it?

[sarvaje]

List of hints:

• hint-apple-touch-icons
• hint-meta-theme-color