Use HarmonyOS Sans on OpenHarmony

[HarmonyOS Sans], which despite the name is also available in
[OpenHarmony](https://gitee.com/openharmony/utils_system_resources),
is really the default and only font on OpenHarmony besides some
language specific NotoSans fonts.

[HarmonyOS Sans]: https://developer.huawei.com/consumer/en/doc/design-guides-V1/font-0000001157868583-V1

src/source.rs

@@ -52,8 +52,12 @@ pub use crate::sources::fs::FsSource as SystemSource;
 const DEFAULT_FONT_FAMILY_SERIF: &'static str = "Times New Roman";
 #[cfg(any(target_family = "windows", target_os = "macos", target_os = "ios"))]
 const DEFAULT_FONT_FAMILY_SANS_SERIF: &'static str = "Arial";
+#[cfg(target_env = "ohos")]
+const DEFAULT_FONT_FAMILY_SANS_SERIF: &str = "HarmonyOS Sans";
 #[cfg(any(target_family = "windows", target_os = "macos", target_os = "ios"))]
 const DEFAULT_FONT_FAMILY_MONOSPACE: &'static str = "Courier New";
+#[cfg(target_env = "ohos")]
+const DEFAULT_FONT_FAMILY_MONOSPACE: &str = "HarmonyOS Sans";
 #[cfg(any(target_family = "windows", target_os = "macos", target_os = "ios"))]
 const DEFAULT_FONT_FAMILY_CURSIVE: &'static str = "Comic Sans MS";
 #[cfg(target_family = "windows")]
@@ -63,9 +67,19 @@ const DEFAULT_FONT_FAMILY_FANTASY: &'static str = "Papyrus";
 
 #[cfg(not(any(target_family = "windows", target_os = "macos", target_os = "ios")))]
 const DEFAULT_FONT_FAMILY_SERIF: &str = "serif";
-#[cfg(not(any(target_family = "windows", target_os = "macos", target_os = "ios")))]
+#[cfg(not(any(
+    target_family = "windows",
+    target_os = "macos",
+    target_os = "ios",
+    target_env = "ohos"
+)))]
 const DEFAULT_FONT_FAMILY_SANS_SERIF: &str = "sans-serif";
-#[cfg(not(any(target_family = "windows", target_os = "macos", target_os = "ios")))]
+#[cfg(not(any(
+    target_family = "windows",
+    target_os = "macos",
+    target_os = "ios",
+    target_env = "ohos"
+)))]
 const DEFAULT_FONT_FAMILY_MONOSPACE: &str = "monospace";
 #[cfg(not(any(target_family = "windows", target_os = "macos", target_os = "ios")))]
 const DEFAULT_FONT_FAMILY_CURSIVE: &str = "cursive";