Migrate to es module / es6 import

[remmel]

Can be checked at https://remmel.github.io/Depthkit.js/examples/simple.html

Related with #36 1.

Here no build system / bundler, because this is a lib is that so usefull (KISS) ? (however depthkit version with vite is here : https://github.com/remmel/Depthkit.js/tree/vite)
the only advantages could be :

• generate UMD files for people who don't want to use a bundler neither es6 (even old browser handle it with es-module-shims); but old script way - dephkit.umd.cjs
• merging in one file (1 http request) for people who are using es6 in browser (without build system) - dephkit.js
• having hot reload system for developers who are improving dephtkit
• being able to import rgbd.vert directly (I import here rgbd.vert.js instead, no such difference : https://github.com/juniorxsound/Depthkit.js/pull/39/files#diff-726e612d5b0a728d897b4cd1d890c2c900326c517f55e1479c273b050193816e )
• use typescript or use new not yet browser supported js features