fix: silence unexpected cfg confition name warning

[FrankenApps]

Currently when compiling the crate, four warnings like the one below appear when using Rust 1.80 and higher:

warning: unexpected `cfg` condition name: `frb_expand`
 --> src/api/init.rs:1:1
  |
1 | #[flutter_rust_bridge::frb(init)]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: using a cfg inside a attribute macro will use the cfgs from the destination crate and not the ones from the defining crate
  = help: try referring to `flutter_rust_bridge::frb` crate for guidance on how handle this unexpected cfg
  = help: the attribute macro `flutter_rust_bridge::frb` may come from an old version of the `flutter_rust_bridge_macros` crate, try updating your dependency with `cargo update -p flutter_rust_bridge_macros`
  = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
  = note: this warning originates in the attribute macro `flutter_rust_bridge::frb` (in Nightly builds, run with -Z macro-backtrace for more info)

The reason is explained in this flutter_rust_bridge issue and also in this official Rust Blog post.

I went with the general recommendation and added the section below to the Cargo.toml file:

[lints.rust]
unexpected_cfgs = { level = "warn", check-cfg = ['cfg(frb_expand)'] }