Select SelectionMode

[jachin]

We need a new SelectionMode. We could call it something like SingeSelectSimple or SelectMimic.

The idea is that it's behavior should mimic that of a regular <select> tag.

A couple of differences between this and a SingleSelect CustomHtml <much-select>:

• no filtering as the user types
• an option is always selected, if there isn't an initial selected value provided the first option is selected
• no custom options
• no multi select

It the attribute API gets a little tricky. Right now we have SelectionMode being handled by multi-select being present or not.

We probably need to convert that to an attribute like selection-mode with the possible values being:

• single-select (default)
• multi-select
• select-mimic

And also support the multi-select attribute but deprecate it.