Feature: add support for impl const Trait syntax

[usmanovbf]

This PR introduces support for parsing constant trait implementations, specifically the const impl and impl const syntaxes. This resolves the parsing error reported in issue #1972 and aligns syn with the experimental syntax introduced by RFC 2632 (Const Trait Impls).

Changes included in this PR:

• AST Modification: Added a new constness: Option<Token![const]> field to the syn::ItemImpl struct.
• Parsing Logic (Parse): Updated the parser in src/item.rs to correctly recognize and consume the const keyword. It handles both const impl Trait and impl const Trait variations to ensure robust compatibility with the experimental compiler features.
• Token Serialization (ToTokens): Updated the implementation to assure the const token is properly preserved and written back into the token stream during code generation.
• Auto-generated AST Traversal: Updated all generated visitor and folder modules (clone.rs, debug.rs, eq.rs, fold.rs, hash.rs, visit.rs, visit_mut.rs) to account for the new constness field.
• Testing: Added comprehensive test coverage in tests/test_item.rs to verify successful parsing, token round-tripping, and correct behavior when combined with other modifiers like default and unsafe.

SemVer Note:
Please note that adding a public field to ItemImpl is a breaking change for users relying on exhaustive pattern matching. I defer to the maintainers on the appropriate milestone or breaking release batch for this PR.

[usmanovbf]

There are failed tests in CI.
Do I need just to add them to EXCLUDE section in the tests/repo/mod.rs:

  "library/core/src/slice/index.rs",
  "library/core/src/str/traits.rs",

?