Allow any identifier as format arg name

Allow any identifier as format arg name

Previously:

    error: invalid format string: invalid argument name `_x`
     --> src/main.rs:2:16
      |
    2 |     println!("{_x}", a=0);
      |                ^^ invalid argument name in format string
      |
      = note: argument names cannot start with an underscore

Not supporting identifiers starting with underscore appears to have been
an arbitrary limitation from 2013 in code that was most likely never
reviewed:
https://github.com/rust-lang/rust/pull/8245/files#diff-0347868ef389c805e97636623e4a4ea6R277

The error message was dutifully improved in #50610 but is there any
reason that leading underscore would be a special case?

This commit updates the format_args parser to accept identifiers with
leading underscores.