Add a nightly feature flag to use nightly features

[hamza1311]

There are many nightly features that Yew can benefit from. We should allow consuming those features for users on nightly. At least the following features could be used:

• type_alias_impl_trait
  • Tracking issue for RFC 2515, "Permit impl Trait in type aliases" rust-lang/rust#63063
  • Usage: Callback
• associated_type_defaults
  • Tracking issue for RFC 2532, "Associated type defaults" rust-lang/rust#29661
  • Usage: Component - Implement imperative refs, capturing scopes to child components #2551 (comment)

We also use nightly features for

• examples:
  

  yew/ci/write-optimisation-flags.sh

  Lines 5 to 8 in b82a5ba

  	cat >> .cargo/config.toml << EOF
  	build-std = ["std", "panic_abort"]
  	build-std-features = ["panic_immediate_abort"]
  	EOF

• documentation:
  

  yew/.cargo/config.toml

  Lines 4 to 5 in b82a5ba

  	[unstable]
  	doctest-xcompile = true