V3 - Remove mutable class properties plus other misc changes

[owenconti]

Lot's of stuff here, apologies for the large size. I can break pieces out to individual PRs if needed.

I'll try to summarize the changes:

Select component

• Remove mutableValue and mutableOptions in hopes of simpler and less code in Select.vue
• Added two new computed properties which take the place of mutableValue and mutableOptions: selectedValue and optionList
• selectedValue is always an array (regardless of the multiple prop). This makes working with it easier because we'll always know the type
• optionList is the combination of the options prop plus any pushed tags

Tests:

• I've updated all the tests to pass. However, because we no longer mutate an internal prop when a value is passed, most of the tests were updated to not pass a value prop. By not passing a value prop, vue-select knows it has to manage the selected value internally. There is an internalValue data value which is used to manage the internal value (very much like mutableValue was before, only internalValue is only used in two places)
• Removed the unused karma test

Dev sandbox:

• I've added two more example usages of vue-select to the sandbox. I wanted to have examples of the three ways you can pass the selected value to the component: v-model, :value with @input, and no value (managed internally).

[owenconti]

Removed the onInput prop and replaced with a method: updateValue

[sagalbot]

Huge thanks for this one @owenconti I think it's a big improvement!