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.

CHANGELOG.md

@@ -20,6 +20,7 @@
 
 ## `bat` as a library
 
+- Deprecate `HighlightingAssets::syntaxes()` and `HighlightingAssets::syntax_for_file_name()`. Use `HighlightingAssets::get_syntaxes()` and `HighlightingAssets::get_syntax_for_file_name()` instead. They return a `Result` which is needed for upcoming lazy-loading work to improve startup performance. See #1747 and #1755 (@Enselic)