Introduce MapExtensionToUnknown MappingTarget (#1889)

Co-authored-by: Martin Nordholts <enselic@gmail.com>

CHANGELOG.md

@@ -9,6 +9,7 @@
 
 - Python syntax highlighting no longer suffers from abysmal performance in specific scenarios. See #1688 (@keith-hall)
 - First line not shown in diff context. See #1891 (@divagant-martian)
+- Do not ignore syntaxes that handle file names with a `*.conf` extension. See #1703 (@cbolgiano)
 
 ## Performance
 
@@ -41,6 +42,8 @@
 - Deprecate `HighlightingAssets::syntaxes()` and `HighlightingAssets::syntax_for_file_name()`. Use `HighlightingAssets::get_syntaxes()` and `HighlightingAssets::get_syntax_for_path()` instead. They return a `Result` which is needed for upcoming lazy-loading work to improve startup performance. They also return which `SyntaxSet` the returned `SyntaxReference` belongs to. See #1747, #1755, #1776, #1862 (@Enselic)
 - Remove `HighlightingAssets::from_files` and `HighlightingAssets::save_to_cache`. Instead of calling the former and then the latter you now make a single call to `bat::assets::build`. See #1802 (@Enselic)
 - Replace  the `error::Error(error::ErrorKind, _)` struct and enum with an `error::Error` enum. `Error(ErrorKind::UnknownSyntax, _)` becomes `Error::UnknownSyntax`, etc. Also remove the `error::ResultExt` trait. These changes stem from replacing `error-chain` with `thiserror`. See #1820 (@Enselic)
+- Add new `MappingTarget` enum variant `MapExtensionToUnknown`. Refer to its docummentation for more information. Clients are adviced to treat `MapExtensionToUnknown` the same as `MapToUnknown` in exhaustive matches. See #1703 (@cbolgiano)
+