Add plain_text CE field formatter for string fields

Problem/Motivation

When using custom elements with string or string_long fields, there is no suitable CE field formatter that outputs the raw text value. The auto formatter works but doesn't trim whitespace or strip newlines.

Proposed resolution

Add a new plain_text CE field formatter that:

• Only applies to string and string_long field types
• Outputs the raw field value directly as a CE attribute (no render pipeline)
• Trims leading/trailing whitespace
• Optionally strips newlines and collapses multiple spaces into one
• Optionally truncates via Unicode::truncate() with word-safe and ellipsis options

The formatter bypasses Drupal's rendering system entirely, setting the value directly on the custom element. This avoids any HTML escaping or markup wrapping that is inappropriate for JSON API output.