IgniteUI / igniteui-webcomponents / 31701023658

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fix(select): anchor keyboard navigation to the current selection (#2326)

Focusing a select that already had a selection and pressing ArrowUp without
opening it cleared the selection, leaving the placeholder behind, instead of
moving to the previous item.

`_activeItem` - the item navigation moves from - was maintained by hand and
only ever assigned by `_selectItem`. Every other path into selection reaches
`_setSelectedItem` directly: the `value` property setter, and `firstUpdated`
resolving a `selected` attribute on an item, where assigning `defaultValue`
routes through that same setter while the component is still pristine and the
follow-up `_selectItem` then short-circuits on an already selected item. Both
left `_activeItem` unset, so `getPreviousActiveItem` was handed `undefined`,
`items.indexOf(undefined)` gave -1, the loop from -2 never ran, and it
returned `items[-1]`. `_selectItem(undefined)` is the "clear everything" path,
so the value was wiped. ArrowDown survived the same input by luck - its loop
starts at 0 and returned the first item.

`_activateItem` is now the only writer of `_activeItem` and is folded into
`_setSelectedItem` and `_clearSelectedItem`, so every path that touches the
selection keeps the two in step. The navigation helpers return `T | undefined`
and handle a missing or detached starting point explicitly - forwards from the
top, backwards from the bottom - and navigating with nowhere to go is a no-op;
clearing the selection is left to the callers that mean it. `igc-dropdown`
shares those helpers but routes their result through a guarded path, so it is
unaffected.

Also in the same area:

- type ahead threw a TypeError on the first character typed whenever
  `_activeItem` was unset, and treated a printable key held with Ctrl/Cmd/Alt
  as a search character, preventing its default so the shortcut never reached
  the browser
- choosing the already selected item from the open list left it open. The
  early return for an unchang... (continued)

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