HighlightingAssets: Make .syntaxes() and syntax_for_file_name() failable

Or rather, introduce new versions of these methods and deprecate the old ones.

This is preparation to enable robust and user-friendly support for lazy-loading.
With lazy-loading, we don't know if the SyntaxSet is valid until after we try to
use it, so wherever we try to use it, we need to return a Result. See discussion
about panics in #1747.

tests/no_duplicate_extensions.rs

@@ -26,7 +26,7 @@ fn no_duplicate_extensions() {
 
     let mut extensions = HashSet::new();
 
-    for syntax in assets.syntaxes() {
+    for syntax in assets.get_syntaxes().expect("this is a #[test]") {
         for extension in &syntax.file_extensions {
             assert!(
                 KNOWN_EXCEPTIONS.contains(&extension.as_str()) || extensions.insert(extension),