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.

src/pretty_printer.rs

@@ -235,7 +235,9 @@ impl<'a> PrettyPrinter<'a> {
     }
 
     pub fn syntaxes(&self) -> impl Iterator<Item = &SyntaxReference> {
-        self.assets.syntaxes().iter()
+        // We always use assets from the binary, which are guaranteed to always
+        // be valid, so get_syntaxes() can never fail here
+        self.assets.get_syntaxes().unwrap().iter()
     }
 
     /// Pretty-print all specified inputs. This method will "use" all stored inputs.