Optimize build speed

[Hyperkid123]

This PR introduces some changes to the webpack build process in order to improve the build speed.

Build time changes

command	time no cache	time cache
build	68s	33s
build:js	60s	28s

Watch results
The initial build takes around 28s. After that, the watch mode re-builds the bundle in a couple of seconds. We no longer have to wait for 20+ s in watch mode after each change.

Changes

• removed hard minimize:true from webpack config
  • this was causing that even dev version of the build was minimizing the code which lead to very long build times
  • minimize property is now based on the mode argument that is passed via npm scripts
• use TerserWebpack plugin as minimizer
  • this is one of the most performant webpack minimizers
  • minimizing is done in parallel and these time savings
  • should be building better source maps ™️
• use HardSourceWebpackPlugin cache
  • this should be included by default in webpack 5
  • the cache significantly speeds up the watch build
  • we should probably set up Travis to store and read this cache to improve the CI speed we can do that later @karelhala @ryelo what do you think about that?

[codecov-commenter]

Codecov Report

Merging #948 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #948   +/-   ##
=======================================
  Coverage   52.21%   52.21%           
=======================================
  Files          53       53           
  Lines        1038     1038           
  Branches      211      211           
=======================================
  Hits          542      542           
  Misses        396      396           
  Partials      100      100           

[karelhala]

Trying the watch mode and it's blazingly fast! Thank you!