fix(Key-Value Editor): UX improvements and bugfix for empty state

[gatzjames]

Highlights:

• Fixes a bug where editing the key/value of the empty state field would trigger an infinite re-render. Also simplified the code and logic for showing the empty state placeholder.
• Switching to another input field doesn't require to click outside now
• Improved highlight styles
• Clicking on an option from the auto-complete keeps the focus to the input and adds the value as expected

Closes INS-4001

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ Error: Expected fetch controller: :r3v: Array.forEach(<anonymous>) View Issue

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