For position named arguments lint, underline both curly brackets when inserting and don't add $ to formatting names in messages

[PrestonFrom]

A comment asked why we only underline the closing curly brace when inserting (https://github.com/rust-lang/rust/pull/99660/files/1b2e05e212f9e2d50fb35817a80136018fbea4ba#r932952838) a named argument. This was originally because that's where it gets inserted -- but on reflection, this is potentially confusing. So underline both.

Also, in the lint message, the formatting argument names are emitted with a dollar sign.
this formatting argument uses named argument precision$ by position
This is also potentially confusing because the actual name does not contain it.

[PrestonFrom]

@rustbot claim