Get rid of `underscore` package

[dr-dimitru]

• [x] Remove underscore dependency
• [x] Search projects for usage of _.
• [x] Create a list of used underscore's helpers
  • [x] isString
  • [x] isEmpty
  • [x] isFunction
  • [x] isObject
  • [x] defaults
  • [x] isRegExp
  • [x] omit
  • [x] reduce
  • [x] keys
  • [x] extend
  • [x] pick
  • [x] isArray
  • [x] each
  • [x] difference
  • [x] last
  • [x] clone
  • [x] toArray
  • [x] values
  • [x] map
  • [x] every
• [x] Create internal helper to cover all used underscore's helpers
• [x] Migrate codebase from _. to internal helper
• [x] Run auto-tests
• [x] Run tests on demo apps
• [x] Version bump
• [x] Publish & Release

[dr-dimitru]

Done in v3.6.0