Add Plane Detection API sample

[bialpio]

Add plane detection API sample.

Things to note:

• The sample perma-links to the .png file from ARCore Android SDK repository and mentions that this is the case in the comment, along with the link to the license - I'm not sure if this is the right way to go here. I could also add the file directly to our repository, in which case we should probably also include a copy of the license somewhere.
• The sample is a heavily modified version of three.js' ar hit test sample - I have mentioned that in the comment and linked to the three.js' license.
• The sample includes a JavaScript-side hit test implementation to showcase that plane detection API allows the sites to implement a synchronous hit-test if they choose so.
• The sample also leverages Anchors API for object placement, and includes Chrome's Origin Trial token.

[bialpio]

@toji / others, PTAL. This should be roughly the same example that we had in Chrome repository, but uses three.js instead of cottontail for rendering and should have less debug information to look less like a tech demo / debug page.