src/printer.rs: Simplify Plain Text fallback code

By forwarding the task to find the `Plain Text` syntax to `assets`. Not only does
the code become simpler; we also get rid of a call to `self.get_syntax_set()`
which is beneficial to the long term goal of replacing `syntaxes.bin` with
`minimal_syntaxes.bin`.

Note that the use of `.expect()` is not a regression in error handling. It was
previously hidden in `.find_syntax_plain_text()`.

src/assets.rs

@@ -245,7 +245,10 @@ impl HighlightingAssets {
         }
     }
 
-    fn find_syntax_by_name(&self, syntax_name: &str) -> Result<Option<SyntaxReferenceInSet>> {
+    pub(crate) fn find_syntax_by_name(
+        &self,
+        syntax_name: &str,
+    ) -> Result<Option<SyntaxReferenceInSet>> {
         let syntax_set = self.get_syntax_set()?;
         Ok(syntax_set
             .find_syntax_by_name(syntax_name)