Improve position named arguments lint underline and formatting names

For named arguments used as implicit position arguments, underline both
the opening curly brace and either:
* if there is formatting, the next character (which will either be the
  closing curl brace or the `:` denoting the start of formatting args)
* if there is no formatting, the entire arg span (important if there is
  whitespace like `{  }`)

This should make it more obvious where the named argument should be.

Additionally, in the lint message, emit the formatting argument names
without a dollar sign to avoid potentially confusion.

Fixes #99907

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