Major restructuring of theme/syntax handling

tests/no_duplicate_extensions.rs

@@ -19,7 +19,7 @@ fn no_duplicate_extensions() {
         "sass",
     ];
 
-    let assets = HighlightingAssets::new();
+    let assets = HighlightingAssets::from_binary();
 
     let mut extensions = HashSet::new();
 

tests/syntax_detection.rs

@@ -18,7 +18,7 @@ struct SyntaxDetectionTest {
 impl SyntaxDetectionTest {
     fn new() -> Self {
         SyntaxDetectionTest {
-            assets: HighlightingAssets::new(),
+            assets: HighlightingAssets::from_binary(),
             syntax_mapping: SyntaxMapping::new(),
             temp_dir: TempDir::new("bat_syntax_detection_tests")
                 .expect("creation of temporary directory"),