Make bat::PrettyPrinter::syntaxes() iterate over new bat::Syntax struct (#2222)

We can't keep `syntect::parsing::SyntaxReference` as part of the public
API, because that might prevent us from bumping to syntect 6.0.0 without
also bumping bat to v2.0.0, once we reach v1.0.0.

So introduce a new stripped down struct `Syntax` and return that
instead. Let it be fully owned to make the API simple. It is not going
to be in a hot code path anyway.

I have looked at all code of our 27 dependents but I can't find a single
instance of this method being used, so this change should be safe for
v1.0.0.

tests/test_pretty_printer.rs

@@ -0,0 +1,16 @@
+use bat::PrettyPrinter;
+
+#[test]
+fn syntaxes() {
+    let printer = PrettyPrinter::new();
+    let syntaxes: Vec<String> = printer.syntaxes().map(|s| s.name).collect();
+
+    // Just do some sanity checking
+    assert!(syntaxes.contains(&"Rust".to_string()));
+    assert!(syntaxes.contains(&"Java".to_string()));
+    assert!(!syntaxes.contains(&"this-language-does-not-exist".to_string()));
+
+    // This language exists but is hidden, so we should not see it; it shall
+    // have been filtered out before getting to us
+    assert!(!syntaxes.contains(&"Git Common".to_string()));
+}