Fix: unused_variables warnings were silently suppressed

[usmanovbf]

Description

This PR fixes #446, resolving the issue where unused_variables warnings were silently suppressed for fields in structs and enums deriving thiserror::Error.

The Problem

Previously, the Display trait implementation generated by the macro unpacked all fields of a given variant and applied a global #[allow(unused_variables)] directive to the entire match block. While this successfully prevented compiler warnings for fields not referenced in the #[error("...")] format string, it had the unintended side effect of masking legitimate dead code and unused variable warnings within the user's own data structures.

The Solution

Instead of relying on a blanket allow directive, this PR implements selective variable binding:

1. Format String Parsing: During the attribute parsing phase (impl/src/fmt.rs), the macro now scans the format string and its arguments to track exactly which fields are referenced (including .field and .0 syntax). These are collected in a new used_field_names set.
2. Selective Pattern Generation: When building the match arm patterns (impl/src/expand.rs), fields that are present in used_field_names are bound to their normal identifiers. Fields that are not referenced in the format string are bound to the wildcard pattern (e.g., field_name: _ for named fields, or simply _ for unnamed fields).
3. Restoring the Lint: With the unused fields correctly ignored via _, the unused_variables exception has been safely removed from the #[allow(...)] attribute over the match block.

As a result, the Rust compiler's standard static analysis is restored for the user's types.

Testing

Added a UI test (tests/ui/unused_variables.rs) with #![deny(unused_variables)]. This ensures via trybuild that the compiler correctly emits errors for unused fields, safeguarding against future regressions of this behavior.

Thank you for your time reviewing this!

[usmanovbf]

@dtolnay there are test in tests/test_display.rs, e.g.

thiserror/tests/test_display.rs

Lines 28 to 37 in 99e8a6c

	#[test]
	fn test_braced_unused() {
	#[derive(Error, Debug)]
	#[error("braced error")]
	struct Error {
	extra: usize,
	}
	assert("braced error", Error { extra: 0 });
	}

where some variables are already unused.
I propose to introduce some migrational period for developers who use thiserror to get rid of the unused variable errors from their code, e.g. for 6 months. And for now it should be enabled by special feature, e.g. features = ["unused_variables"]
If you agree, I will supress the errors in tests/test_display.rs by enabling this feature by default in order to make tests green and throwing a warning with the migration period disclaimer for developers