feat: add 'left' and 'right' options to dropdownPosition (ng-select#1990

)

* feat: add 'left' and 'right' options to dropdownPosition

feat: add clearSearchOnAdd to NgSelectConfig (ng-select#1975)

feat(styles): added variables in default theme to allow customization… (

ng-select#1892)

perf: reduce change detection cycles by listening the `mousedown` in …

…the root zone (ng-select#1929)

This PR reduces change detection cycles for the `ng-dropdown-panel` by replacing
`HostListener` with `fromEvent` outside of the zone. Calling `preventDefault()`
on the event doesn't require Angular to run `ApplicationRef.tick()`. Note that the
`HostListener` wraps the actual listener under the hood into the internal Angular function
which runs `markDirty()` before running the actual listener (the decorated class method).

fix: add 1 pixel for reliable triggering scrollToEnd (ng-select#1877)

fix: update peer dep declaration ng 12 (ng-select#1920)

feat: upgrade to angular 12 (ng-select#1903)

BREAKING CHANGE: update to Angular 12

feat: allow nullish compareWith (ng-select#1857)

There are 2 reasons to allow setting compareWith to null/undefined.

1. Reverting the set compareWith - possible the user of the ng-select might want to remove the previously set compareWith to allow default behaviour in combination with bindValue possibly.
2. Extensibility - we have a wrapper around the ng-select that provides custom styles, adds error messages. Not all use cases use compareWith, but some do. Now it is impossible to redirect the [compareWith] @input without setting the default. That hinders the bindValue default behaviour.

feat: upgrade to angular 11 (ng-select#1844)

BREAKING CHANGE: update to Angular 11
* remove tsickle (related: angular/angular#37221)

The closure compiler advanced optimizations has been disabled with
angular/angular#37221 and therefore the tsickle pass is not going to be
very beneficial and blocks updating to Angular 11. When closure compiler
is better supported this configuration can be added back.

* build ng-option-highlight with ng-select

feat: support bindLabel in global config (ng-select#1840)