chore: drop dependency to proc-macro-error2

[tisonkun]

No description provided.

[Copilot]

Pull request overview

This PR removes the proc-macro-error2 dependency from the stacksafe-macro proc-macro crate by switching to syn::Result/syn::Error for error reporting, and updates the workspace dependency set accordingly.

Changes:

• Replace proc-macro-error2 abort-based error handling with syn::Error propagation and into_compile_error() emission.
• Introduce an internal stacksafe_impl returning syn::Result<TokenStream> and adapt parsing to syn::parse2 / Parser::parse2.
• Update workspace dependency declarations and lockfile to drop proc-macro-error2 and bump related crate versions.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
stacksafe-macro/src/lib.rs	Reworks macro entrypoint and error handling to remove proc-macro-error2 usage.
stacksafe-macro/Cargo.toml	Drops proc-macro-error2, adds direct proc-macro2 dependency.
Cargo.toml	Updates workspace dependency list to remove proc-macro-error2 and pin proc-macro2 and other crates.
Cargo.lock	Reflects dependency graph changes: removes proc-macro-error2 packages and updates resolved versions.

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