Future incompatibility with Rust RFC 3373: Avoid non-local definitions in functions

[Nerixyz]

CI currently passes, but might break with the 2024 edition. Example:

warning: non-local `impl` definition, they should be avoided as they go against expectation
    --> twitch_oauth2/src/tokens/errors.rs:16:35
     |
  16 | #[derive(thiserror::Error, Debug, displaydoc::Display)]
     |                                   ^^^^^^^^^^^^^^^^^^^
     |
     = help: move this `impl` block outside the of the current constant `_DERIVE_Display_FOR_ValidationError`
     = note: an `impl` definition is non-local if it is nested inside an item and may impact type checking outside of that item. This can be the case if neither the trait or the self type are at the same nesting level as the `impl`
     = note: one exception to the rule are anon-const (`const _: () = { ... }`) at top-level module and anon-const at the same nesting as the trait or type
     = note: this lint may become deny-by-default in the edition 2024 and higher, see the tracking issue <https://github.com/rust-lang/rust/issues/120363>
     = note: the derive macro `displaydoc::Display` may come from an old version of the `displaydoc` crate, try updating your dependency with `cargo update -p displaydoc`
     = note: this warning originates in the derive macro `displaydoc::Display` (in Nightly builds, run with -Z macro-backtrace for more info)

• This depends on Future incompatibility with Rust RFC 3373: Avoid non-local definitions in functions yaahc/displaydoc#46