Recognize ixx as part of the cpp suffix (#26333)

Adds "ixx" as path suffix to be recognized for c++. 

> ixx documentation
https://learn.microsoft.com/en-us/cpp/cpp/modules-cpp?view=msvc-

I've also added it to the icon file. 

Release Notes:

- N/A

crates/theme/src/icon_theme.rs

@@ -85,7 +85,7 @@ const FILE_SUFFIXES_BY_ICON_KEY: &[(&str, &[&str])] = &[
     ("coffeescript", &["coffee"]),
     (
         "cpp",
-        &["c++", "cc", "cpp", "cxx", "hh", "hpp", "hxx", "inl"],
+        &["c++", "cc", "cpp", "cxx", "hh", "hpp", "hxx", "inl", "ixx"],
     ),
     ("crystal", &["cr", "ecr"]),
     ("csharp", &["cs"]),