Add core conversion impls and optional interop with smol_str/compact_str

[Copilot]

This PR expands CowStr/InlineStr interoperability by adding missing core conversion traits and feature-gated impls for widely used compact string crates. It keeps default behavior unchanged while enabling opt-in integrations.

• Core trait coverage

  • Added AsRef<[u8]> for both CowStr and InlineStr.
  • Added From<Box<str>>, From<&String>, and From<InlineStr> for CowStr.
  • Added From<CowStr> for InlineStr.

• Feature-gated popular crate interop

  • Introduced new optional features:
    • smol_str → dep:smol_str
    • compact_str → dep:compact_str
  • Added bidirectional conversions for both CowStr and InlineStr with:
    • smol_str::SmolStr
    • compact_str::CompactString

• Docs and API surface updates

  • Updated crate-level feature docs and README to document new optional integrations.

• Illustrative usage

  use moos::{CowStr, InlineStr};
  
  #[cfg(feature = "smol_str")]
  {
      let s = smol_str::SmolStr::new("abc");
      let cow = CowStr::from(s.clone());
      let back = smol_str::SmolStr::from(cow);
  
      let inline = InlineStr::from(s);
      let back2 = smol_str::SmolStr::from(inline);
  }
  
  #[cfg(feature = "compact_str")]
  {
      let s = compact_str::CompactString::from("abc");
      let cow = CowStr::from(s.clone());
      let back = compact_str::CompactString::from(cow);
  }