feat(select): add --highlight-height variable

[brandyscarney]

Issue number: internal

---

What is the current behavior?

In v7, using the legacy syntax, the height of the highlight on an item could be adjusted using the --highlight-height variable. This variable was not added to select and therefore would not work using the modern syntax.

What is the new behavior?

Adds the --highlight-height variable so that developers can customize this height in md mode. Since the highlight element is not added for ios mode, this variable won't do anything for ios. Note that this diverges from the v7 behavior, where setting --highlight-height enabled the highlight for ios. A design document outlining this has been proposed here: https://github.com/ionic-team/ionic-framework-design-documents/pull/252

Does this introduce a breaking change?

• [ ] Yes
• [x] No