Add value property to mdc-select JS implementation

[traviskaufman]

Our mdc-select JS component is missing the value getter /setter property, which is part of the HTMLSelectElement API. We should implement this in order to have better parity with the native DOM counterpart.

[traviskaufman]

Impl:

• new getValueForOptionAtIndex() adapter method (see below)
• getValue() foundation method which returns the text content
• value getter in component which proxies to getValue

[traviskaufman]

Since we don't use actual <option> elements, I actually don't think we need another adapter method; we can just use getTextForOptionAtIndex instead.

From mdn:

value
The content of this attribute represents the value to be submitted with the form, should this option be selected. If this attribute is omitted, the value is taken from the text content of the option element.

Since our list item elements cannot have a value attribute, and the option role has no value ARIA attribute, I think it makes sense here to just use the textContent of the selected item.

[traviskaufman]

chatted with @yeelan0319 about this and we decided on the following impl:

• use getValueForOptionAtIndex(), and have it return the value of the id attribute for the selected option, or the text content of the selected option if no id.