refactor(transfer): align with select & monorepo structure

Fixes #50

This commit does several things:

* Use strings as selected values
* Use strings to store which options are highlighted
* Removes jest test abstractions
* Simplifies internal names
* Replaces common.js files with files whose names match their content
* Moves the Transfer component from core to widgets
* Adds the .js extension to all import statements where applicable
* Provides the correct disabled & dataTest props to the re-order up
* button's icon

BREAKING CHANGE: The Transfer component now expects options to be passed in as objects, not as children. Custom components can be provided via the optionComponent prop for all options or via the component property on an individual option.

BREAKING CHANGE: The Transfer component now expects strings as selected
values instead of option objects.

BREAKING CHANGE: The Transfer component is now part of widgets

.eslintignore

@@ -1,2 +1,2 @@
-**/src/locales/*
 **/cypress/assets
+**/src/locales/*

cypress/integration/Transfer/display-order/index.js

@@ -61,9 +61,9 @@ When('the user selects multiple options', () => {
         })
         .each($option => cy.wrap($option).clickWith('ctrl'))
         .then($options => {
-            cy.wrap($options.toArray().map(extractOptionFromElement))
-                .as('selectedOptions')
-                .then(console.log.bind(null, 'selectedOptions'))
+            cy.wrap($options.toArray().map(extractOptionFromElement)).as(
+                'selectedOptions'
+            )
         })
 
     cy.get('{transfer-actions-addindividual}').click()
@@ -150,7 +150,6 @@ Then(
             () => cy.get('{transfer-sourceoptions} {transferoption}'),
             () => cy.get('@deselectedOptions')
         ).should(([win, $selectableSourceOptions, deselectedOptions]) => {
-            console.log('deselectedOptions', deselectedOptions)
             const selectableSourceOptions = $selectableSourceOptions
                 .toArray()
                 .map(extractOptionFromElement)
@@ -169,9 +168,6 @@ Then(
                         value === deselectedOption.value
                 )
 
-                if (!result) {
-                    console.log('deselectedOption', deselectedOption)
-                }
                 return result
             })
             expect(hasAllOptions).to.equal(true)
@@ -205,8 +201,6 @@ Then(
                 .slice(transferredOptions.length * -1)
                 .map(extractOptionFromElement)
 
-            console.log('transferredOptions', transferredOptions)
-            console.log('lastSelectedOptions', lastSelectedOptions)
             expect(transferredOptions).to.eql(lastSelectedOptions)
         })
     }