Remove outdated DOM utilities and simplify marker SVG

[mourner]

• Remove DOM.setTransform(el, ...) in favor of el.style.transform = ....
• Remove DOM.remove in favor of el.remove() (supported everywhere except IE11).
• Remove DOM.addEventListener/DOM.removeEventListener in favor of el.addEventListener (options / passive listeners supported everywhere except IE11).
• Introduce DOM.createSVG utility in place of DOM.createNS which we only used for SVG.
• Overhaul default marker SVG generation code, removing redundant elements and simplifying code so that default markers are now more performant while looking slightly nicer — previously the shadow was approximated by 8 ellipses of varying opacity, now it's one with a gradient.
• Consequently, reduce the gzipped bundle size by 688 bytes.

Marker before:

Marker after:

Launch Checklist

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• n/a write tests for all new functionality
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Improved default marker performance.</changelog>

[SnailBones]

👍 Looks good