printf: Add indexing to format strings

[sargas]

This PR implements the ability for printf (and the general formatting functionality in uucore) to have "conversion specifications" that explicitly say which argument to use. This comes from the Single Unix Specification and is tested for in GNU's tests/printf/printf-indexed test.

To handle arbitrary positions, I've changed the iterator-based approach to a slice of FormatArgument's, and wrapped that in a struct to keep track of the "next" argument to grab (as well as how the positions should be offset when repeatedly processing the format string

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/printf/printf-indexed is no longer failing!

[sylvestre]

Congrats! The gnu test tests/printf/printf-indexed is no longer failing!

kudos!

[Copilot]

Pull Request Overview

This PR implements support for positional conversion specifications in printf formatting by changing the argument handling from an iterator to a new FormatArguments struct, along with corresponding changes in parsing and format‐writing routines.

• Added positional specifier support in format parsing (e.g. "%1$d", "%2$s", etc.).
• Refactored argument handling using a stateful FormatArguments struct to support both sequential and positional argument access.
• Updated and added tests to cover various scenarios with positional and mixed access patterns.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/by-util/test_printf.rs	Updated tests and added new test for positional format specifiers
src/uucore/src/lib/features/format/spec.rs	Modified Spec enum and parsing functions to record argument positions
src/uucore/src/lib/features/format/num_format.rs	Adjusted pattern matching for CanAsterisk to account for new positional syntax
src/uucore/src/lib/features/format/mod.rs	Changed argument handling by using FormatArguments instead of iterators
src/uucore/src/lib/features/format/argument.rs	Introduced and refactored FormatArguments to support positional indexing and batching
src/uu/printf/src/printf.rs	Refactored printf to work with the new FormatArguments and batching logic