item-start and item-end make more sense for RTL
* refactor(item): replaced item-left with item-start
replaced item-right with item-end
* style(item): fix spacing
* fix(item): add backwards support for left/right in ng-content
* fix(item): deprecated old variables, not breaking change
* fix(scss): fix variable name
* fix(item): old attributes deprecated support
Internal refactor completed in order to improve tree shaking and dead
code removal. The public API, with an exception to ion-slides, has
stayed the same. However, internally many changes were required so
bundlers could better exclude modules which should not be bundled.
Ultimately most changes resorted to removing references to `window` or
`document`, or a module that referenced one of those.
BREAKING CHANGES
ion-slides was refactored to remove the external dependencies, and
rewritten in TypeScript/ES6 modules to again improve tree shaking
abilities.
