Fix: allow using the full Private Use Area for custom fonts (#3509)

The ignored characters are used in some custom fonts.

for example: the \u{F0FF} is used as `cleaning_services` in
MaterialIcons-Regular.ttf

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Closes <https://github.com/emilk/egui/issues/THE_RELEVANT_ISSUE>.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/epaint/src/text/fonts.rs

@@ -335,6 +335,23 @@ impl FontDefinitions {
             families,
         }
     }
+
+    /// List of all the builtin font names used by `epaint`.
+    #[cfg(feature = "default_fonts")]
+    pub fn builtin_font_names() -> &'static [&'static str] {
+        &[
+            "Ubuntu-Light",
+            "NotoEmoji-Regular",
+            "emoji-icon-font",
+            "Hack",
+        ]
+    }
+
+    /// List of all the builtin font names used by `epaint`.
+    #[cfg(not(feature = "default_fonts"))]
+    pub fn builtin_font_names() -> &'static [&'static str] {
+        &[]
+    }
 }
 
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