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.

internal/compiler/generator/rust.rs

@@ -2929,6 +2929,10 @@ fn compile_builtin_function_call(
             quote!(#(#a)*.as_str().parse::<f64>().unwrap_or_default())
         }
         BuiltinFunction::StringIsFloat => quote!(#(#a)*.as_str().parse::<f64>().is_ok()),
+        BuiltinFunction::StringIsEmpty => quote!(#(#a)*.is_empty()),
+        BuiltinFunction::StringCharacterCount => {
+            quote!( sp::UnicodeSegmentation::graphemes(#(#a)*.as_str(), true).count() as i32 )
+        }
         BuiltinFunction::ColorRgbaStruct => quote!( #(#a)*.to_argb_u8()),
         BuiltinFunction::ColorHsvaStruct => quote!( #(#a)*.to_hsva()),
         BuiltinFunction::ColorBrighter => {