Expand variable picker functionality

[engram-design]

Referring to the "variable picker" for being able to select field values, submission values, or other static content:

We want to expand this functionality:

• Right-clicking on these variables presents a context menu for additional options (for some variables)
• Searchable and replaceable (to swap it with another variable without having to remove and re-add)
• Option-based fields (Dropdown, Checkboxes, Radio Buttons) will have the option to use either label or value
• Element-based fields can pick either CP URL or front-end URLs for elements
• URL/Link fields can choose whether to render HTML, or the raw URL

This will also be tied into under-the-hood changes for using the UID of fields, rather than their handle.

Visually, it'll look similar to: