String: Add .is-empty and .character-count properties

Introduce two new properties for string in .slint:
- .is-empty: Checks if a string is empty.
- .character-count: Retrieves the number of grapheme clusters
  https://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries

These additions enhance functionality and improve convenience when working with string properties.

api/rs/slint/Cargo.toml

@@ -195,6 +195,8 @@ log = { workspace = true, optional = true }
 
 raw-window-handle-06 = { workspace = true, optional = true }
 
+unicode-segmentation = "1.12.0"
+
 [target.'cfg(not(target_os = "android"))'.dependencies]
 # FemtoVG is disabled on android because it doesn't compile without setting RUST_FONTCONFIG_DLOPEN=on
 # end even then wouldn't work because it can't load fonts

api/rs/slint/private_unstable_api.rs

@@ -227,5 +227,6 @@ pub mod re_exports {
     pub use once_cell::race::OnceBox;
     pub use once_cell::unsync::OnceCell;
     pub use pin_weak::rc::PinWeak;
+    pub use unicode_segmentation::UnicodeSegmentation;
     pub use vtable::{self, *};
 }