fix(item): add input highlight using an absolute div (#15856)

Adds the following CSS properties to item:

```
--highlight-color-focused
--highlight-color-valid
--highlight-color-invalid
--highlight-height
```

This also fixes an issue where we were showing the highlight on items
with no lines, and shows inset vs full properly. Adds documentation and tests for input focus.

fixes #14036 fixes #9639 fixes #14952 closes #15690

core/src/components/input/input.tsx

@@ -324,7 +324,6 @@ export class Input implements ComponentInterface {
     return {
       class: {
         ...createColorClasses(this.color),
-
         'in-item': hostContext('ion-item', this.el),
         'has-value': this.hasValue(),
         'has-focus': this.hasFocus