Add 'anchor' option for Markers

[anandthakker]

Following up from #4751, it would be nice to add an 'anchor' option for Marker, similar to the one in Popup:

mapbox-gl-js/src/ui/popup.js

Lines 37 to 42 in f11b764

	* @param {string} [options.anchor] - A string indicating the popup's location relative to
	* the coordinate set via {@link Popup#setLngLat}.
	* Options are `'top'`, `'bottom'`, `'left'`, `'right'`, `'top-left'`,
	* `'top-right'`, `'bottom-left'`, and `'bottom-right'`. If unset the anchor will be
	* dynamically set to ensure the popup falls within the map container with a preference
	* for `'bottom'`.

[jmandel1027]

Happy to take a crack at it if no one has it in their queue n_n

Do we want to export a MarkerOptions type as done in popup as well?

mapbox-gl-js/src/ui/popup.js

Lines 22 to 27 in f11b764

	export type PopupOptions = {
	closeButton: boolean,
	closeOnClick: boolean,
	anchor: Anchor,
	offset: Offset
	};

[anandthakker]

@jay-manday I don't think anyone's claimed it yet, so please feel free!

Do we want to export a MarkerOptions type as done in popup as well?

Couldn't hurt!

[jmandel1027]

Hi @anandthakker

I thiiiink i have things working as needed though would love a review of my work before making a PR to the master branch on your end. I did some digging on the origins of the issue and was a pretty cool learning experience! 🌀💡🎉✨

Heres the work done on my fork:
jmandel1027@47d1243

I also went ahead and manually wrote the test for the marker file too. n_n

[andrewharvey]

Hi @jay-manday nice to see you implementing this! Feel free to go ahead and open that PR as that's where the review typically takes place, even if you feel it's not 100% ready, you can just mention it's a WIP.

The options for Popup's anchor are 'top', 'bottom', 'left', 'right', 'top-left', 'top-right', 'bottom-left', and `'bottom-right'. I think that's what we should implement for the Marker anchor.

[jmandel1027]

Hi @andrewharvey, totally! I'll make a PR and we can further discuss the work so far. Thanks for pointing me in the right direction as to the desired anchor options.

I think I went down a wrong turn with my reasoning, while investigating how the marker renders onto the map I made the assumption that we wanted to anchor the marker via pixel values rather than css. I will definitely will factor this into my PR. n_n